This is called the Fibonacci sequence. The last two numbers in the sequence are added together to produce the next. Therefore, following 8 and 13 are:
8 + 13 = 21
13 + 21 = 34
21 + 34 = 55
34 + 55 = 89
55 + 89 = 144
They appear to be 13 and 17
1, -3, -7
1 5 9 13 17 21
The next three numbers in the sequence are 23, 25, and 27.
The Fibonacci sequence is infinite. The next two terms are 89 and 144.
They appear to be 13 and 17
1, -3, -7
1 5 9 13 17 21
The next three numbers in the sequence are 23, 25, and 27.
13 21 34 55
125 13 23 43 53...
The Fibonacci sequence is infinite. The next two terms are 89 and 144.
Since the terms of the given sequence differ by different numbers, then the terms would start a new pattern, so that the other term of the sequence is 23.16 26 21 13 23 18 10...
There are many possible answers. One obvious one is 13, the next number in the Fibonacci Sequence that yields the golden mean.
-17, -21 Pattern is subtract 4.
The next number in the sequence 5-7-13-31 is 69.
13 This is because each term of the sequence is determined by adding the 2 previous terms of the sequence. This particular sequence is called the Fibonacci Sequence, and has special properties. See related link.