Videos :
Math Magic
by Michael Stevens (Vsauce, 2016年02月16日).
Little Fibs and
more
by Colm Mulcahy (Numberphile, 2016年06月02日).
This is one of the better tricks of its kind, because the effect of reversing the digits isn't obvious to most people at first... If the 3-digit number reads abc, it's equal to 100a+10b+c and the second step gives the following result:
| (100a+10b+c) - (100c+10b+a) | = 99 | a-c |
The quantity | a-c | is between 2 and 9, so the above is a 3-digit multiple of 99, namely: 198, 297, 396, 495, 594, 693, 792 or 891. The middle digit is always 9, while the first and last digits of any such multiple add up to 9. Thus, adding the thing and its reverse gives 909 plus twice 90, which is 1089, as advertised.
The trick will become boring (or obvious) if the same table is used repeatedly. Thus, a new table must be provided each time. Several online implementation do this quite effectively, with nice graphics. Examples:
A Game from milaadesign.com
Magic Gopher (British Council)
How fast can you discover the secret which makes this work?
[ is always divisible by 9.'+
'
The same symbol appears at all positions that are multiples of 9.');
return false">Answer ]
Effect : The magician hands out a 3 or 4-digit integer chosen by a spectator in a previous part of the show. Using a pocket calculator, another spectator multiplies that number by some secret 3-digit number which he chooses freely and keeps for himself. The result is a 6 or 7-digit number. The spectator withholds one of those digits and reveals all the others in a random order. The magician then reveals the withheld digit!
The trick is based on arithmetic modulo 9, which is what underlies the process of casting out nines from an integer, which is very familiar to schoolchildren (at least it used to be). Casting out nines is a quick way to obtain the remainder when an integer N is divided by 9 (the key observation is that 10 and all the powers of 10 leave a remainder of 1, therefore, a number and the sum of its digits leave the same remainder). See elsewhere on this site for other divisibility rules.
Secret : The number handed out by the magician is a multiple of 9 (Art Benjamin takes it from a random list of perfect squares coming from a earlier stage of the show; each of those has one chance in 3 of being divisible by 9 and the earlier stage went on until a "good" number came up). The result is therefore a multiple of 9 and the sum of its digits is a multiple of 9. When all the digits but one are revealed, the last one is thus known modulo 9. This does reveal it unless it's either a zero or a nine. In that ambiguous case, the magician will guess it to be a 9 and will almost always be right because people will rarely skip a zero when they are told to skip any digit they like. If you'd rather not take any chances at all, then instruct people to skip a nonzero digit...
A quick way to obtain the missing digit mentally is to first work out the remainder modulo 9 of the sum of the digits you are given (just add together the digits and repeat, if needed, until you obtain a single-digit result). Subtract the result from 9. Done, unless you get zero (in which case you'd guess "9" instead, as previously discussed).
Many variants of this trick can be devised based on any obscure process which produces a multiple of 9. Here is one:
Ask a spectator to pick any 4-digit number and to consider the number obtained by reading it backwards. Let the spectator secretly subtract the lesser number from the larger one, add 54 and multiply the result by a 3-digit number freely chosen by the spectator...
Ask how many digits there are in the final result and ask the spectator to keep one nonzero digit secret and to reveal the other digits in scrambled order. (Count ostensibly on your fingers how many digits you are given to make sure you're only missing one.)
You may then call the remaining digit with perfect accuracy.
Videos :
Arthur Benjamin does Mathemagic
at TED (February 2005)
Casting out nines with a deck of 40 cards
by ScamSchool's Brian Brushwood (crediting MIT Rob).
The following effect can be repeated as many times as needed to convince the spectators that you can read their minds with perfect accuracy as they pick one of three choices.
Put three cards face-up on the table. Ask a spectator to choose one mentally and remember its position. Flip the three cards over.
Turn around and instruct the spectator to show the other spectators which card he has chosen, then have him switch the two other cards behind your back.
Now face the table again and instruct the spectators to switch cards as many times as they wish in front of your eyes.
Reveal the card originally chosen by the spectator.
Before you flip the cards and turn around, remember which card is in the middle. When you face the table again, focus of the card which is now in the middle and keep track of its position as spectators move cards around.
Flip that card.
If this is the card you had memorized, simply announce that this is the card the spectators have chosen (this is so because the spectator clearly hasn't switched the middle card behind your back in this case). Otherwise, the chosen card can be neither the one you had memorized nor the one you're now seeing. It's the third card.
Video : Scam School by Brian Brushwood, crediting Seth Rovner (2010年06月16日).
In a 1992 TV show, David Copperfield turned simple-minded mathematical properties into something wonderful, for an audience who was (skillfully) led to expect magical things to happen.
Copperfield first asks you to take N steps forward and N steps back. It doesn't matter what N is, does it? Later, he says to go halfway around a circle in whichever direction you choose (another type of irrelevant choice).
Regardless of the details of that show, it should be clear that a magician can only make predictions about outcomes which do not depend on the choices of his many spectators. However, surprisingly many people want to believe in some irrational explanation. This is what really scares me.
Besides the visual effects and the drama, the challenge in designing such a collective effect is also to devise instructions that everyone can follow...
The teacher tells the class that a crowd can be driven to think about the same thing; very few people will escape the mental picture shared by all others...
Each student in the class is asked to think about a small number and is then instructed to perform the following operations silently.
The teacher then announces to a puzzled classroom that their collective thinking must have gone wrong, since "there are no grey elephants in Denmark"...
Well, there are elephants in Denmark: At this writing, the home of Kungrao (M), Surin (F) and Tonsak (F) is the Copenhagen Zoo...
The Denmark is thus the only obvious \'choice\'.'+
'
How many animal names start with \'E\' ?');
return false">trick works in most parts of the World,
but I wonder how many students from the Caribbeans would think
of an "ostrich in Dominica" instead.
Just a joke!
The 4! = 24 ways of showing 4 given cards in order would not be enough to differentiate among the remaining 48 cards of the pack. However, since we may choose what card is offered for guessing, we have an additional choice among 5. The resulting 120 possible courses of action are more than enough to convey the relevant information. Here's one practical way to do so:
Consider two cards of the same suit (among 5 cards, at least one such pair exists). Let's call them the base card and the hidden card, in whichever order makes it possible to go from the base card to the hidden card card by counting at most 6 steps clockwise on a circle of the 13 possible values. (King is followed by Ace, Ace is followed by 2, 3, 4, etc.)
We offer the hidden card up for "guessing". By revealing the base card first, we are telling the suit of the hidden card and we also set the point where a count of up to 6 "clockwise" steps is to begin to determine the hidden card.
The order in which the remaining 3 cards are presented can be used to reveal this count, as there are 6 possible permutations of 3 given cards. Using some agreed-upon ordering of the cards in a deck, we hold a high card (H), a medium card (M) and a low card (L). Some arbitrary code is used, like:
LMH = 1 ; LHM = 2 ; MLH = 3 ; MHL = 4 ; HLM = 5 ; HML = 6
This trick is credited to Dr. William Fitch Cheney, Jr. (Fitch the Magician, 1894-1974) who earned the first math Ph.D. ever awarded by MIT (1927).
The puzzle is presented in the 1960 book of Wallace Lee entitled Math Miracles (chapter 14, as quoted by Martin Gardner) and was popularized by the magician Art Benjamin in 1986. It was used in a 1994 job interview and subsequently appeared on the rec.puzzles newsgroup, where Bob Vesterman posted the particular solution presented above (1994年04月25日).
In 1995, Robert Orenstein implemented Vesterman's encoding for online play at www.anamorph.com/docs/ct/cards.html (a dead link resuscitated from the 2007 archives, courtesy of deadURL.com, on 2010年04月25日). For many years, that page was apologizing for having "temporarily" shut down its (terse) interactive features, since 2002年08月15日. Fortunately, that part was revived in the same terse form, by Tom Ace, an admirer of the trick who happens to be a software engineer.
The Best Card Trick
(PDF) by Michael Kleber.
Mathematical Intelligencer 24 #1 (Winter 2002).
Fitch Cheney's Five-Card Trick
by Colm Mulcahy (MAA Horizons, Feb. 2003).
The previous article deals with k=4, n=5, d=52. The case k=3, n=8, d=13 is called Devil's Poker : The Devil chooses 5 cards of a single suit and you present 3 of the remaining 8 cards one by one to an Angel who must guess the Devil's hand, using a prior convention between you and the Angel.
We have k! C(n,k) = n!/(n-k)! possible actions to reveal one of C(d-k,n-k) compatible possibilities. This task is only possible if the former exceeds the latter, which means that n!(d-n)! must be greater than or equal to (d-k)! .
That means k = n-1 in the case considered by Michael Kleber in the aforementioned Mathematical Intelligencer article (PDF). In that case, the above inequality boils down to:
d < n! + n
Kleber goes on to show that this necessary condition is sufficient to establish a working strategy.
This trick is attributed to the physicist Martin David Kruskal (1925-2006). It illustrates a statistical feature which is amazing enough when one first encounters it. Here's one way to present the effect:
If we use a regular deck of cards, we either remove the face cards or attribute to them the same value (1) as aces. Beforehand, a player chooses secretly a special number N from 1 to 10. As the cards from the deck are revealed one by one, the player counts cards and considers the N-th card revealed to be his new special number and keeps counting N cards from that one, and so forth... All told, only a few cards are thus singled out as special. The majority are not...
Yet, toward the end of the deck the dealer (the magician) can confidently point out that one particular card is "special"...
The same trick can be demonstrated by a clever dealer who just looks at the cards before dealing them and announces that a specific card (which may then be flipped over and replaced in the deck) will turn out to be special. You may play this version online with a computer which (honestly) shuffles the deck. Allow yourself to be baffled a few times before reading on...
Well, the explanation is simply statistical. For the sake of simplicity, let's consider the related case of an infinite sequence of cards, each bearing a positive integer n with probability pn .
[画像: Come back later, we're still working on this one... ]
Two subsequences extracted with the above rules from an infinite sequence of digits (0 to 9) will eventually coincide, because if they coincide once they coincide forever (think about it).[画像: Come back later, we're still working on this one... ]
Kruskal Number-Spelling Illusion (18:26) by Kevin (Vsauce2, 2020年02月26日)
In the May 1999 issue of Games Magazine, Martin Gardner published the following puzzle, among a small collection of some magic tricks with numbers. It involves the first sentences of the US Declaration of Independence :
You are instructed to pick any word in the first (red) section of the text. Then, skip as many words as there are letters in your chosen word. For example, if you picked the fourth word ("Course") you have to skip 6 words ("of human Events, it becomes necessary") to end up on the word "for"... Iterate the same process, by skipping as many words as there are letters in the successive words you land on.
What's the first word you encounter in the last (green) section? Answer: God. Always. (The sequence would continue with the words: descent, that, causes.)
The "magic" is based on the Kruskal principle discussed above... You will ultimately land on God by starting with most words in the middle (yellow) section. The words that do work have been underlined for you. You may check that this underlining is correct by working it out (backwards) for yourself, starting with the last yellow words ("and", "of") which do land on God in one step. As any word which leads to an underlined word gets underlined itself, almost all words in the yellow section end up being underlined. This includes the first 17 words of that yellow section. Since all words of the red section have less than 17 letters, that solid chunk of underlined words can't be jumped over and, therefore, all paths starting in the red section will ultimately lead to the word "God" in the green section. (Actually, any word up to the word "Station" is a valid beginning of a sequence which ends up on the word "God".)
Kruskal Count by Doctor Douglas (2007年04月01日)
We'll only discuss here the most famous system of its kind. It's named after Si Stebbins (William Coffrin) who presented it in 1898 as his own invention, although it had been published as early as 1612. Even earlier, Horatio Galasso wrote about a closely related system in 1593 (just using a numeric progression by constant increments of 4, instead of 3 for the Si Stebbins). More complicated schemes of the same family feature four distinct increments (one per suit; the only restriction being that their sum shouldn't be a multiple of 13).
The ordering illustrated above and presented at right is also revealed at the end of a video posted by Furrukh Jamal presenting two related magic tricks.
Such a deck can be cut many times, but not shuffled (seasoned illusionists could use false shuffling ).
The value of the Nth card from the top (face down) is:
x = B + 3 N (mod 13)
Here, B is the value of the bottom card. The following numerical convention is used (modulo 13):
With the numerical code for suits given at the bottom of our main table, if S is the suit of the bottom card, then the suit of the Nth card is simply:
y = S + N (mod 4)
For example, if the bottom card is the jack of diamonds (B=11, S=0) then the tenth card (N=10) is a deuce (since 11+3.10 is 41, which is equal to 2 modulo 13). It's the deuce of hearts because 0+10 is equal to 2 modulo 4.
One trick is to have a spectator cut the deck. You
secretly look at the bottom card and call the card
3 units higher in the next suit (from the
"CHaSeD" sequence
Clubs, Hearts, Spades, Diamonds) before revealing the top card.
Conversely, the position N of the card x of suit y can be obtained from the Chinese Remainder Theorem (a result N=0 would denote the bottom card). Since 3N is x-B modulo 13, N is -4(x-B) modulo 13 (HINT: -4x3 is -12 or +1 modulo 13). With that value of N modulo 13 and the value of N modulo 4 (namely y-S) we may apply our explicit formula to solve the Chinese Remainder Problem and obtain N modulo 52 = 4x13, namely:
N = 13 bezout (13,4) (y-S) - 4 bezout (4,13) 4 (x-B)
Since bezout (13,4) = 1 (mod 4) and bezout (4,13) 4 = 1 (mod 13) , that expression boils down to the following easy-to-memorize formula:
The existence of such a formula makes the above far more flexible than other stacking schemes which lack arithmetic regularity (including the infamous "Eight Kings CHaSeD" stack, which is merely based on the mnemonic sentence: "Eight Kings threa-ten to save nine fair ladies for one sick knave" for the order 8K3T2795Q4A7J).
For example, if the bottom card is the jack of diamonds (B=11, S=0) then the queen of hearts (x=12, y=2) is at the following position (modulo 52):
N = 13 (2-0) - 4 (12-11) = 22
The king of spades is at N = 13 (3-0) - 4 (13-11) = 31
The queen of diamonds is at N = 13 (0-0) - 4 (12-11) = -4 = 48
The ace of clubs is at N = 13 (1-0) - 4 (1-11) = 53 = 1 (Isn't it?)
Preparation : Here's a quick method to arrange the deck as above:
Mother of All
Card Tricks Revealed by Furrukh Jamal (Video)
Memorized
Deck Online Toolbox by Scott Cram (at "Grey Matters")
"SCAM School" videos :
Centuries Old Magician's SECRET Card Trick!
(courtesy of Diamond Jim Tyler).
Learn an Insanely Easy Card Trick
(Spell: COLOR, NUMBER, MATCHING-CARD)
The "Enigma" card trick performed by Andy Field revealed by Jay Mismag822 "The card-trick teacher"
Some traditional magic age cards forgo the numbers 61, 62 and 63 (so that only 29 or 30 numbers per card are required, which are printed in a 5 by 6 pattern, with or without a star in the 30th position). Full-range cards (with 32 numbers printed on each card) are more satisfying. Here they are:
16 17 18 19
20 21 22 23
24 25 26 27
28 29 30 31
48 49 50 51
52 53 54 55
56 57 58 59
60 61 62 63
08 09 10 11
12 13 14 15
24 25 26 27
28 29 30 31
40 41 42 43
44 45 46 47
56 57 58 59
60 61 62 63
04 05 06 07
12 13 14 15
20 21 22 23
28 29 30 31
36 37 38 39
44 45 46 47
52 53 54 55
60 61 62 63
02 03 06 07
10 11 14 15
18 19 22 23
26 27 30 31
34 35 38 39
42 43 46 47
50 51 54 55
58 59 62 63
01 03 05 07
09 11 13 15
17 19 21 23
25 27 29 31
33 35 37 39
41 43 45 47
49 51 53 55
57 59 61 63
Effect : A spectator thinks of a number (up to 63) and tells you on what cards it is.
You call the exact number!
Secret : The weight of each card is the smallest number printed on it. Any number is equal to the sum of the weights of the cards it appears on. For example:
52 =わ 32 +たす 16 +たす 4
This is just a straight consequence of binary numeration. Each card actually shows all the numbers which have a "1" in their respective binary representations at a given position. The binary representation of 52 being 110100, it appears on 3 cards and is equal to the sum of the 3 relevant powers of 2. Voilà.
Magic Age Cards (1ドル.29 Party Trick) | Number Guessing Game at Cut-the-Knot
This is my own improvement (2009年01月14日) over traditional "age cards".
The introduction of black and red colors allows a larger range of numbers (80 instead of 63) using fewer cards (just 4 cards instead of 6).
Effect : A spectator thinks of a number (up to 80) and tells you its color (red or black) on each card where it appears.
You call the exact number!
Secret : For each color called by the spectator, add the smallest number of the same color on the card. The total will be the secret number.
For example:
52 = 1 + 6 + 18 + 27
Those cards are based on ternary numeration: In base 3, all numbers less than 81 are represented by 4 digits or less. Each card shows the 54 numbers which have a nonzero digit at a specific ternary position. If the digit is 1, the number is listed in red. If the digit is 2, the number is listed in black.
This is a classic no-brainer. Deal any odd number of cards up to 27 in three equal piles (this means you're dealing 15, 21 or 27 cards, according to taste). Ask what pile the chosen card belongs to and collate the cards so the chosen pile is in the middle. Deal and collate again in the same way. Deal one last time. The chosen card will be in the middle of the selected row. Reveal it in whatever dramatic way you like...
For a very fast effect, use just 9 cards and deal only twice (although the underlying math for this 2-step trick becomes rather obvious).
Video 1
|
Video 2
Beautiful Card Trick (13:14)
by Matt Parker (Numberphile, 2012年11月26日)
For once, let's explain the effect before presenting a video performance.
One method of eliminating half the cards in a face-down deck is to flip-over every other card, starting with the topmost one, to form two piles and get rid of the face-up pile...
1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17, ... 48,49,50,51,52.
The first elimination gets rid of the odd-numbered cards and reverses the order of the even-numbered ones:
52,50,48,46,44,42,40,38,36,34,32,30,28,26,24,22,20,18,16,14,12,10,8,6,4,2.
The second step leaves 13 cards face-down, in the following order:
2, 6, 10, 14, 18, 22, 26, 30, 34, 38, 42, 46, 50.
The third step eliminates every other card and reverses the order again:
46, 38, 30, 22, 14, 6.
The final elimination leaves three cards face-down: 6, 22 and 38. In other words, the first of the "Final Three" was initially at position 6 (i.e., with 5 face-down cards on top of it) and the other two appeared thereafter at regular intervals of 15 intervening cards...
In the video presentation below, the number 15 is prominent and the number 5 is the result of subtracting 4 from 9 = 52-(10+1+15+1+15+1)... There are also two "false cuts" to give an impression of randomness.
Video : The Final 3 "Amazing Card Trick" by Mismag822.
Multiply your chosen number by any odd number and multiply the number you did not choose by any even number. Add those two products together.
From that result, how can a magician have the same parity.'); return false">determine which number was chosen?
In a perfect faro shuffle of an even number of cards, the deck is split into equal halves which are then interweaved.
There are two ways to do the interweaving. In an out shuffle, both the top card and the bottom card are unchanged In a so-called in shuffle neither is (the top card becomes second and the bottom card becomes next-to-last).
Out-shuffling 2n+2 cards is equivalent to in-shuffling the inner 2n cards.
[画像: Come back later, we're still working on this one... ]
Faro Shuffle Tutorial | Faro Shuffle Explained
When the deck consists of an odd number of cards, the deck is split into a pack of n+1 cards and a pack of n cards.
If the larger pack is on the bottom, then the bottom card always remains unchanged and we are simply faced with a faro shuffling of only the 2n top cards. So, only the case where the top pack is larger need be considered. For an out shuffle that case is equivalent to an out shuffle of 2n+2 cards (in an outshuffle of an even number of cards, the bottom card stays in place). All told, the only case where the faro shuffling of an odd number of cards does not reduce trivially to the shuffling of an even number of cards is the following one:
Faro shuffling with an odd deck.
In-shuffling of 2n+1 cards, cutting n+1 cards from the top.
In such a shuffle, there's a pair of adjacent cards from the middle of the pack which remain adjacent at the bottom of the pack after the shuffle. It's much less regular than the other type of faro shuffling. Yet, some patterns appear:
The number s of such shuffles needed to return a deck of n cards to its original state is a complicated function of n. Remarkably, if n is 3 units below a power of 2, then s is a simple quadratic function of the exponent (usually, the ratio s/n is then much smaller than for any lesser values of n).
If n is the number of cards above a given card in the top half of the deck, then the number of cards above that same card becomes:
Let's encode a sequence of perfect shuffles with a binary number Q where a "0" bit corresponds to an out-shuffle and a "1" to an in-shuffle. For example, an in-shuffle followed by two out-shuffles corresponds to "100" (which is the binary expansion of the integer Q = 4).
If Q is less than the number of cards in the deck, the above remark can be used to show (by induction on the number of shuffles) that a sequence of shuffles of code Q moves the topmost card Q places down (i.e., after shuffling, there are Q cards above the card which was originally on top).
Video : 52-Card Perfect Shuffles by Jason Davison (in "Numberphile" by Brady Haran, 2015年03月27日).
This classical trick can be done with ordinary coins (each side is either heads or tails ). However, it's simpler and more spectacular with coins whose sides are easy to tell apart from a distance. Othello/Reversi pieces (discs) are ideal for this: They have a white side and a black one...
Put all the discs on the table, flip some of them over, shuffle them. Ask your spectators to do the same. Explain the difference between shuffling the coins (sliding only) and flipping them over. Now, turn around and tell the spectators to shuffle the coins behind your back ("no flipping") then announce that you will separate the whole mess into two heaps containing the same number of white discs "using your sense of touch alone".
You do just that, very quickly (using both hands to go faster). Then put your hands up in the air and turn around (in that order) to check with the spectators that you've accomplished the improbable.
Do it several times and the improbable will look like the impossible.
Before turning your back, you count the number W of white discs.
What you do behind your back is simply pick W discs randomly and flip them over as you put them flat on the table to form a separate heap.
Consider any heap of W discs taken from a set that originally contained W white discs and any number of black ones.
If x is the number of white discs in that heap, then there are W-x black discs in it, which is precisely the number of white discs that you left in the rest of the set.
If you flip over all the discs in your heap, there are now as many white discs in it (namely, W-x) as in the rest of the set. Halmos
Two heaps containing the same number of white discs.
Videos : Maths Puzzle: Coins
&
Maths Puzzle: Coins (Solution)
by James Grime (SingingBanana).
The basic principle featured here is named after Norman L. Gilbreath (a computer scientist and amateur magician, born in March 1936) who is a;so remembered for Gilbreath's conjecture which he rediscovered and popularized, 80 years after its first formulation by François Proth (1851-1879).
Gilbreath published a first version (concerning only the black or red colors of cards) in The Linking Ring, 38, 5 (July 1958). Introducing himself as a math-major UCLA student, Gilbreath named his effect "Magnetic Colors". Although this had been known to some magicians before, it was received as a major innovation by the Magic community. So much so that, eight years later, an entire issue of The Linking Ring (June 1966) would be devoted to Gilbreath magic. In it, Gilbreatth himself presented a great generalization, now known as the second principle of Gilbreath, on which we are focusing here...
The general principle is that a certain type of shuffling which ends with an honest riffle-shuffle (performed by a spectator) will always preserve one particular aspect of a cyclic deck with respect to a separation of the deck into n equal classes (e.g., 2 colors, 4 suits, 13 values): Even after such a shuffle, every "slice" of n cards still contains one card of every class !
The Gilbreath Shuffle : Starting from a cyclic deck (cut any number of times) the first step of a proper Gilbreath shuffle consists in separating the deck into two piles by dealing out one of them, thereby reversing the order of the cards in it (you may pretend that you're counting the cards to "make sure" the piles are exactly even, although that's actually irrelevant). The second step is to let a spectator riffle-shuffle those two piles together once.
Why it works : After a Gilbreath shuffle, the situation is exactly identical to what would be obtained if we had built the deck by successively picking a card either from the top or the bottom of the original deck (at random). Since slices of n cards are ordered alike at the top or the bottom of our cyclic deck, we always obtain a full slice of different cards after n picks, irrespective of the random choices made. (The same basic situation repeats after a whole number of slices have been so obtained.) QED
In the case n = 2 (for black/red colors only, as presented by Gilbreath in 1958) it's an overkill to deal out one pile completely... It suffices to make sure that the bottom cards in the two piles have different colors (while preserving alternating colors). This easy version is most often used by magicians (see video clips in the footnotes below).
Here's my way to apply the full Gilbreath principle with a 52-card deck. Slices of sizes 13 and 4 are used simultaneously in this example:
Ask a spectator to take 12 cards off the top of the shuffled deck and to secretly remove one of them. The other 11 cards are then exposed. You stress that you clearly can't tell their card for sure without seeing all the other 51 cards. Yet, you guarantee to call it with perfect accuracy, by just peeking at the four top remaining cards... Then, you do just that!
As the deck is initially cyclically stacked, the second Gilbreath principle applies: The correct suit is the only suit which appears just 2 times (not 3) among the first 11 cards shown. The value of the hidden card is the only value missing from the first 12 cards revealed (the 11 cards originally shown and the first card turned over from the top of the remaining deck).
Yes, only the top card is needed; the extra ones just help conceal the secret!
Also, the first 11 cards allow you to determine the correct suit and leave a choice of only two values. The correct value is whichever of those two differs from the value of the top card (to be revealed next). Thus, you can be ready to call the spectator's card very fast when the top cards are turned over (all at once). This speed-flourish enhances the effect's impact.
The
First Norman Invasion (MAA, August 2005)
|
The
Second Norman Invasion (MAA, August 2006)
Wikipedia :
Gilbreath shuffle
Videos :
Diagonal Attraction by Mismag822
|
Scam School by Brian Brushwood
Shuffling Card Trick by Jason Davison
(Numberphile by Brady Haran, 2016年03月02日).
This trick works by relating the number of cards in several piles to the values of their top cards. The video by Brian Brushwood (link in the footnote) demonstrates the effect and shows how to perform it (same thing really, for this particular effect) but doesn't provide an explanation... Here's one:
Well, in the last stage of the trick, each of the three face-down piles on the table contains 14-n cards, where n is the value (between 1 and 13) of the top card. If you deal out the number of cards indicated by that top card for two piles, each of them will have consumed a total of 14 cards. Therefore, from the original 52 cards, 24 cards are left (52-28) which are either in your hand or in the remaining pile of 14-n cards, where n is the value of the face-down top card, which you have to determine...
So, if you have removed 10 cards from your hand at the outset, there's a total of 14 cards left. As 14-n of those are in the "unknown" pile, you are holding exactly n cards. Counting them reveals the value of the pile's top card. QED
Many variants of the same mathematical trick start with the construction of several piles containing 14-n cards ( n being the value of each pile's own top card). The key point is that, when one such pile is finally chosen, the number of cards outside of it is simply 38+n. Thus, the real challenge in designing such an effect is to come up with some interesting method to get rid of 38 cards before the final count (in a sufficiently obscure way).
(14-n) + (38+n) = 52
Video : The easiest card-trick ever (Scam School) by Brian Brushwood.
The secret is to assign a unique number to each card an know what the total for the whole deck is. Mentally keep the running total of all the cards which have been shown. When only one card remains, its number is obtained by subtracting that total from the known total for the whole deck.
If we use numbers from 1 to 52, the total for the whole deck is:
(52+1) 52 / 2 = 1378
The formula behind that calculation was discovered by Gauss when he was seven years old.
In practice, it's easier to assign cards in each suit values from 0-12, 20-32, 40-52 and 60-72. The grand total for that scheme is 1872.
Better yet, use modular arithmetic modulo 4 or 13 to add separately the suits (the total for the whole deck is 2 modulo 4) and the values (the total for the whole deck is 0 modulo 13).
Video : Mike Sexton on Stu Ungar Counting Cards (3:02) by PartyPoker (2014年09月10日).
An effect in search of a good way to present it.
Proizvolov Identity (1985)
by Vyacheslav Proizvolov.
Video :
Proizvolov Identity (3:02)
by James Grime (2019年06月25日).
Presentation : Behind the magician's back, a spectator rolls three dice and adds the pips, then choses one die and adds the number on its bottom face. Finally, the chosen die is rolled again and the result is added to the total.
Effect : The magician turns around and names the total secretly computed.
Secret : Just add 7 to the total which finally shows.
Proof : Being opposites on a die, the two unknown numbers add up to 7.
Three-dice trick (7:09) by Ben Sparks (Numberphile, 2021年07月19日).
