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Here the first 100 numbers of an easy sequence:

0,1,0,2,1,4,3,7,6,11,10,16,15,22,21,29,28,37,36,46,45,56,55,67,66,79,78,92,91,106,105,121,120,137,136,154,153,172,171,191,190,211,210,232,231,254,253,277,276,301,300,326,325,352,351,379,378,407,406,436,435,466,465,497,496,529,528,562,561,596,595,631,630,667,666,704,703,742,741,781,780,821,820,862,861,904,903,947,946,991,990,1036,1035,1082,1081,1129,1128,1177,1176,1226

How does this sequence work?

n: 0 1 2 3 4 5 6 7 8 9 10 11 12
 0, 1-1=0, 2-1=1, 4-1=3, 7-1=6, 11-1=10, 16-1=15, 
 0+1=1, 0+2=2, 1+3=4, 3+4=7, 6+5=11, 10+6=16, 15+7=22

• a(0) = 0
• For every odd n (0-indexed), it's a(n-1) + X (where X=1 and increases by 1 every time it's accessed)
• For every even n (0-indexed), it's a(n-1) - 1

Challenge:

One of:

• Given an input integer n, output the n'th number in the sequence.
• Given an input integer n, output the first n numbers of the sequence.
• Output the sequence indefinitely without taking an input (or taking an empty unused input).

Challenge rules:

• Input n can be both 0- or 1-indexed.
• If you output (part of) the sequence, you can use a list/array, print to STDOUT with any delimiter (space, comma, newline, etc.). Your call.
• Please state which of the three options you've used in your answer.
• You'll have to support at least the first 10,000 numbers (10,000th number is 12,497,501).

General rules:

• This is code-golf, so shortest answer in bytes wins.
  Don't let code-golf languages discourage you from posting answers with non-codegolfing languages. Try to come up with an as short as possible answer for 'any' programming language.
• Standard rules apply for your answer, so you are allowed to use STDIN/STDOUT, functions/method with the proper parameters and return-type, full programs. Your call.
• Default Loopholes are forbidden.
• If possible, please add a link with a test for your code.
• Also, please add an explanation if possible.

Test cases:

Pastebin with the first 10,001 numbers in the sequence. Feel free to pick any you'd like.

Some higher numbers:

n (0-indexed) Output:
68,690 589,772,340
100,000 1,249,975,000
162,207 3,288,888,857
453,271 25,681,824,931
888,888 98,765,012,346
1,000,000 124,999,750,000

code-golf number sequence integer