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 A212502 #51 Feb 18 2021 00:45:58 %S A212502 4,8,12,16,24,32,36,48,56,64,72,96,108,112,128,132,143,144,156,168, %T A212502 192,216,224,256,264,272,288,312,324,336,384,392,396,399,432,448,468, %U A212502 496,504,512,527,528,544,552,576,624,648,672,768,779,784,792,816,864 %N A212502 Composite numbers k that divide the imaginary part of (1+2i)^A201629(k). %C A212502 If p is a prime number then p divides the imaginary part of (1+2i)^A201629(p). %C A212502 The numbers of this sequence may be called Fermat pseudoprimes to base 1+2i. %H A212502 Robert Israel, <a href="/A212502/b212502.txt">Table of n, a(n) for n = 1..10000</a> %H A212502 Jose María Grau, A. M. Oller-Marcen, Manuel Rodriguez and D. Sadornil, <a href="http://arxiv.org/abs/1401.4708">Fermat test with Gaussian base and Gaussian pseudoprimes</a>, arXiv:1401.4708 [math.NT], 2014. %p A212502 A201629:= proc(n) if n::even then n elif n mod 4 = 1 then n-1 else n+1 fi end proc: %p A212502 filter:= proc(n) not isprime(n) and type(Powmod(1+2*x, A201629(n), x^2+1, x) mod n, integer) end proc: %p A212502 select(filter, [$2..1000]); # _Robert Israel_, Nov 06 2019 %t A212502 A201629[n_]:=Which[Mod[n,4]==3,n+1,Mod[n,4]==1,n-1,True,n]; Select[1+ Range[1000], ! PrimeQ[#] && Im[PowerMod[1 + 2I, A201629[#], #]] == 0 &] %Y A212502 Cf. A201629, A213337, A212601. %K A212502 nonn %O A212502 1,1 %A A212502 _José María Grau Ribas_, May 19 2012 %E A212502 Definition revised by _José María Grau Ribas_, Oct 12 2013