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 A340867 #8 Mar 18 2021 08:23:18 %S A340867 0,2,1,0,3,0,1,0,3,0,1,2,3,2,1,2,3,0,1,2,1,0,3,0,1,0,1,2,1,0,1,2,1,0, %T A340867 3,2,1,0,3,0,1,2,3,0,1,0,3,0,3,0,1,0,3,2,1,0,3,2,1,0,1,2,3,0,3,2,3,0, %U A340867 1,2,3,2,1,2,3,0,3,2,3,2,3,0,1,2,3,0 %N A340867 a(n) = (prime(n) - a(n-1)) mod 4; a(0)=0. %H A340867 Simon Strandgaard, <a href="/A340867/a340867.png">Visualization</a> %F A340867 a(n) = A008347(n) mod 4. %e A340867 a(1) = ( 2 - 0) mod 4 = 2, %e A340867 a(2) = ( 3 - 2) mod 4 = 1, %e A340867 a(3) = ( 5 - 1) mod 4 = 0, %e A340867 a(4) = ( 7 - 0) mod 4 = 3, %e A340867 a(5) = (11 - 3) mod 4 = 0. %t A340867 a[0] = 0; a[n_] := a[n] = Mod[Prime[n] - a[n - 1], 4]; Array[a, 100, 0] (* _Amiram Eldar_, Jan 30 2021 *) %o A340867 (Ruby) require 'prime' %o A340867 values = [0] %o A340867 Prime.first(50).each do |prime| %o A340867 values << (prime-values[-1]) % 4 %o A340867 end %o A340867 p values %Y A340867 Cf. A008347, A339448. %K A340867 nonn %O A340867 0,2 %A A340867 _Simon Strandgaard_, Jan 24 2021