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 A255904 #24 Sep 08 2022 08:46:11
%S A255904 2,7,61,211,216
%N A255904 Numbers that are neither triangular nor the sum of a positive triangular number and a prime.
%C A255904 Probably finite.
%C A255904 First four terms are primes, and are in A065397.
%C A255904 If it exists, a(6) > 5*10^6. - _Derek Orr_, Mar 12 2015
%C A255904 If it exists, a(6) > 4*10^9. - _Hiroaki Yamanouchi_, Mar 16 2015
%e A255904 a(2) = 7 is in the sequence because 7 is not a triangular number, and cannot be written as a sum of the primes < 7 {2, 3, 5} and one of the positive triangular numbers < 7 {1, 3, 6}.
%o A255904 (Magma) lst:=[]; r:=21; for n in [0..r*(r+1)/2] do if not IsSquare(8*n+1) then f:=0; k:=1; while k*(k+1)/2 lt n-1 do if IsPrime(n-Truncate(k*(k+1)/2)) then f:=1; break; end if; k+:=1; end while; if IsZero(f) then Append(~lst, n); end if; end if; end for; lst;
%o A255904 (PARI) a(n)=k=1;while(n-(t=k*(k+1)/2)>=0,if(isprime(n-t)||n==t,return(k));k++)
%o A255904 n=1;while(n<10^5,if(!a(n),print1(n,", "));n++) \\ _Derek Orr_, Mar 12 2015
%Y A255904 Cf. A065397, A076768.
%K A255904 nonn,fini
%O A255904 1,1
%A A255904 _Arkadiusz Wesolowski_, Mar 10 2015