Hello seqFans, I guess this is a permutation of the natural numbers: « Smallest positive integer not already in sequence with a(n)+a(n+1)+a(n+2)+a(n+3) prime » S = 1,2,3,5,7,4,13,17,9,8,25,11,15,10,... Based on: http://www2.research.att.com/~njas/sequences/A055265 Best, É.