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 A279084 #19 Jan 23 2017 10:01:16 %S A279084 0,1,2,3,11,77,363,1111,2662,24662,26642,111111,246642,11111111, %T A279084 363474363,2664444662 %N A279084 Numbers k such that reverse(T(k)) = T(reverse(k)) where T(k) is the k-th triangular number. %C A279084 The k-th triangular number is T(k) = k*(k+1)/2 = A000217(k). %C A279084 This sequence includes all numbers k such that both k and T(k) are palindromes (A008510); it also includes at least one pair of nonpalindromes (see Example section). It includes 11, 1111, 111111, and 11111111, but not 1111111111. %C A279084 If it exists, a(17) > 10^12. - _Lars Blomberg_, Jan 23 2017 %e A279084 Since reverse(T(11)) = reverse(66) = 66 = T(11) = T(reverse(11)), 11 is in the sequence. %e A279084 Since reverse(T(24662)) = reverse(304119453) = 354911403 = T(26642) = T(reverse(24662)), 24662 is in the sequence (and so is its reverse, 26642). %o A279084 (PARI) rev(n) = eval(concat(Vecrev(Str(n)))); %o A279084 trg(n) = n*(n+1)/2; %o A279084 isok(n) = rev(trg(n)) == trg(rev(n)); \\ _Michel Marcus_, Jan 14 2017 %Y A279084 Cf. A000217, A008510. %K A279084 nonn,base,more %O A279084 1,3 %A A279084 _Jon E. Schoenfield_, Jan 14 2017 %E A279084 a(16) from _Lars Blomberg_, Jan 23 2017