A008510 -id:A008510 - cp's OEIS frontend

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A279084 Numbers k such that reverse(T(k)) = T(reverse(k)) where T(k) is the k-th triangular number.

Original entry on oeis.org

0, 1, 2, 3, 11, 77, 363, 1111, 2662, 24662, 26642, 111111, 246642, 11111111, 363474363, 2664444662

Offset: 1

Jon E. Schoenfield, Jan 14 2017

The k-th triangular number is T(k) = k*(k+1)/2 = A000217(k).
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.
If it exists, a(17) > 10^12. - Lars Blomberg, Jan 23 2017

			Since reverse(T(11)) = reverse(66) = 66 = T(11) = T(reverse(11)), 11 is in the sequence.
Since reverse(T(24662)) = reverse(304119453) = 354911403 = T(26642) = T(reverse(24662)), 24662 is in the sequence (and so is its reverse, 26642).

Cf. A000217, A008510.

rev(n) = eval(concat(Vecrev(Str(n))));
trg(n) = n*(n+1)/2;
isok(n) = rev(trg(n)) == trg(rev(n)); \\ Michel Marcus, Jan 14 2017

a(16) from Lars Blomberg, Jan 23 2017

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