fantam

(5×5)+1=26

(2×6)+1=13

(1×3)+1=4

Prime numbers: ... , 3, 5, 7, 11, 13, 17, 19, 23, ...

$55 - 26 = 19$ ,

while

$26 - 13 = 13$ .

So 19 is considered,17 is skipped, 13 is considered. So we skipped 11 as well:

$13 - 7 = 4$

Ok, just kidding :)

None of these r helpful

To be honest, i had to think for a while before finding the similarity between those values. But actually every number n in this sequence is the product of (n - 1) plus 1 ;)