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 A200514 #10 Jan 03 2016 04:37:11
%S A200514 0,3,4,0,3,0,4,3,0,0,3,16,0,3,4,0,3,0,4,3,0,0,3,16,0,3,4,16,3,40,4,3,
%T A200514 0,0,3,0,0,3,4,16,3,63,4,3,63,0,3,20,0,3,4,20,3,40,4,3,80,0,3,16,0,3,
%U A200514 4,0,3,0,4,3,0,40,3,16,80,3,4,16,3,0,4,3,0
%N A200514 Least m>0 such that n = y^2 - 4^x (mod m) has no solution, or 0 if no such m exists.
%C A200514 To prove that an integer n is in A051206, it is sufficient to find integers x,y such that y^2 - 4^x=n. In that case, a(n)=0. To prove that n is *not* in A051206, it is sufficient to find a modulus m for which the (finite) set of all possible values of 4^x and y^2 (mod m) allows us to deduce that y^2 - 4^x can never equal n. The present sequence lists the smallest such m>0, if it exists.
%H A200514 M. F. Hasler, <a href="/A200514/b200514.txt">Table of n, a(n) for n = 0..688</a>
%e A200514 See A200512.
%o A200514 (PARI) A200514(n,b=4,p=3)={ my( x=0, qr, bx, seen ); for( m=3,9e9, while( x^p < m, issquare(b^x+n) & return(0); x++); qr=vecsort(vector(m,y,y^2-n)%m,,8); seen=0; bx=1; until( bittest(seen+=1<<bx, bx=bx*b%m), for(i=1,#qr, qr[i]<bx & next; qr[i]>bx & break; next(3))); return(m))}
%Y A200514 Cf. A051204-A051221, A200505-A200520.
%K A200514 nonn
%O A200514 0,2
%A A200514 _M. F. Hasler_, Nov 18 2011