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%I #14 Jul 14 2021 02:02:07
%S 1,2,3,11,77,363,1111,2662,111111,246642,11111111,363474363,
%T 2664444662,26644444662,246644446642,266444444662,2466444446642,
%U 3654345456545434563
%N Numbers k such that both k and the k-th triangular number are palindromes.
%D D. Wells, The Penguin Dictionary of Curious and Interesting Numbers. Penguin Books, NY, 1986, 93.
%H Patrick De Geest, <a href="http://www.worldofnumbers.com/subtria.htm">Subsets of palindromic triangulars</a>
%H Patrick De Geest, <a href="http://www.worldofnumbers.com/beautiful.htm">For the last term</a>
%t palQ[n_] := Reverse[x = IntegerDigits[n]] == x; t = {}; Do[If[palQ[n] && palQ[n*(n + 1)/2], AppendTo[t, n]], {n, 1.2*10^7}]; t (* _Jayanta Basu_, May 15 2013 *)
%K nonn,base,more
%O 1,2
%A _Patrick De Geest_
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