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 A139117 #11 Oct 21 2024 04:34:56 %S A139117 3,6,15,28,91,153,190,496,1891,4005,5778,8128,135981,184528,818560, %T A139117 2427706,2602621,5176153,9046131,9783676,46943205,49416711,62871291, %U A139117 198751953,235477551,269340445,990013753,3718970646,6105511756,8718535225,23347768186,286403014380 %N A139117 Triangular numbers (A000217) with indices A000043. %H A139117 Amiram Eldar, <a href="/A139117/b139117.txt">Table of n, a(n) for n = 1..48</a> %F A139117 a(n) = A000217(A000043(n)). %e A139117 a(4) = 28 because A000043(4) = 7 and the 7th triangular number A000217(7) is 28. %t A139117 Map[#*(#+1)/2 &, MersennePrimeExponent[Range[48]]] (* _Amiram Eldar_, Oct 21 2024 *) %Y A139117 Cf. A000217, A000396, A034953. %K A139117 nonn %O A139117 1,1 %A A139117 _Omar E. Pol_, May 08 2008