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 A083571 #24 Jul 02 2025 11:34:15
%S A083571 2,3,5,10,12,72
%N A083571 Numbers k such that A008509(k) is prime.
%H A083571 Patrick De Geest, <a href="https://www.worldofnumbers.com/subtria.htm">Subsets of palindromic triangulars</a>
%t A083571 palQ[n_]:=Reverse[x=IntegerDigits[n]]==x; t={}; c=0; Do[If[palQ[n*(n+1)/2],c=c+1; If[PrimeQ[n],AppendTo[t,c]]],{n,10^3}]; t (* _Jayanta Basu_, May 14 2013 *)
%o A083571 (PARI) ispal(n)=n=digits(n); for(i=1, #n\2, if(n[i]!=n[#n+1-i], return(0))); 1
%o A083571 for(k=1,1e9,if(ispal(k*(k+1)/2),n++;if(isprime(k),print1(n", ")))) \\ _Charles R Greathouse IV_, May 15 2013
%Y A083571 Cf. A008509, A050722, A008510, A050721.
%K A083571 nonn,base,hard,more
%O A083571 1,1
%A A083571 _N. J. A. Sloane_, Jun 14 2003