NSW Number
An NSW number (named after Newman, Shanks, and Williams) is an integer m that solves the Diophantine equation
| 2n^2=m^2+1. |
(1)
|
In other words, the NSW numbers m index the diagonals of squares of side length n having the property that the squares of the diagonal d=sqrt(2)n equals one plus a square number m^2. Such numbers were called "rational diagonals" by the Greeks (Wells 1986, p. 70). The name "NSW number" derives from the names of the authors of the paper on the subject written by Newman et al. (1980/81).
The first few NSW numbers are therefore m=1, 7, 41, 239, 1393, ... (OEIS A002315), which correspond to square side lengths n=1, 5, 29, 169, 985, 5741, 33461, 195025, ... (OEIS A001653). The values indexed by m and n therefore give 2, 50, 1682, 57122, ... (OEIS A088920).
Taking twice the NSW numbers gives the sequence 2, 14, 82, 478, 2786, 16238, ... (OEIS A077444), which is exactly every other Pell-Lucas number.
The first few prime NSW numbers are m=7, 41, 239, 9369319, 63018038201, 489133282872437279, ... (OEIS A088165), corresponding to indices k=1, 2, 3, 9, 14, 23, 29, 81, 128, 210, 468, 473, 746, 950, 3344, 4043, 4839, 14376, 39521, 64563, 72984, 82899, 84338, 85206, 86121, 139160, ... (OEIS A113501).
The following table summarizes the largest known NSW primes, where the indices k correspond via k=(k^'-1)/2 to the indices k^' of prime half-Pell-Lucas numbers that are odd.
Interestingly, the values m/n give every other convergent to Pythagoras's constant sqrt(2).
Explicit formula for m and n are given by
for positive integers k (Ribenboim 1996, p. 367). A recurrence relation for m=S(k) is given by
| S(k)=6S(k-1)-S(k-2) |
(4)
|
with S(0)=1 and S(1)=7.
See also
Pell Number, Pythagoras's ConstantExplore with Wolfram|Alpha
More things to try:
References
Newman, M.; Shanks, D.; and Williams, H. C. "Simple Groups of Square Order and an Interesting Sequence of Primes." Acta Arith. 38, 129-140, 1980/81.Ribenboim, P. "The NSW Primes." §5.9 in The New Book of Prime Number Records. New York: Springer-Verlag, pp. 367-369, 1996.Sloane, N. J. A. Sequences A001653/M3955, A002315/M4423, A077444, A088165, A088920, and A113501 in "The On-Line Encyclopedia of Integer Sequences."Wells, D. The Penguin Dictionary of Curious and Interesting Numbers. Middlesex, England: Penguin Books, p. 70, 1986.Referenced on Wolfram|Alpha
NSW NumberCite this as:
Weisstein, Eric W. "NSW Number." From MathWorld--A Wolfram Resource. https://mathworld.wolfram.com/NSWNumber.html