This is a front-end for the Online Encyclopedia of Integer Sequences, made by Christian Perfect. The idea is to provide OEIS entries in non-ancient HTML, and then to think about how they're presented visually. The source code is on GitHub.
%I A258742 #10 Jun 11 2015 09:37:50
%S A258742 1,3,4,2,6,7,5,9,8,10,11,12,14,13,15,16,17,19,18,20,21,22,24,23,25,27,
%T A258742 26,28,29,31,32,30,33,34,36,37,35,38,39,41,40,42,44,43,45,46,47,49,48,
%U A258742 50,52,51,53,54,55,56,57,58,59,60,62,61,63,64,65,67,66,68,69,71,72,70,73,74,75,76,77,78,79,81
%N A258742 With a(1) = 1, a(n) is the smallest positive number not already in the sequence such that a(n)^2 + a(n-1)^2 is not prime.
%C A258742 Believed to be a permutation of the natural numbers.
%t A258742 f[n_] := Block[{a = {1}, k}, For[k = 2, k <= n, k++, i = 1; While[Or[PrimeQ[i^2 + a[[k - 1]]^2], MemberQ[a, i]], i++]; AppendTo[a, i]]; a]; f@ 120 (* _Michael De Vlieger_, Jun 10 2015 *)
%o A258742 (PARI) v=[1]; n=1; while(n<100, if(!isprime(n^2+v[#v]^2)&&!vecsearch(vecsort(v), n), v=concat(v, n); n=0); n++); v
%Y A258742 Cf. A055266, A100208.
%K A258742 nonn
%O A258742 1,2
%A A258742 _Derek Orr_, Jun 08 2015