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 A122963 #7 Jun 13 2016 07:49:09 %S A122963 10,21,45,55,105,120,231,253,325,351,561,595,630,741,780,1081,1225, %T A122963 1326,1540,1653,1711,1953,2145,2415,2775,3003,3403,3655,3741,3828, %U A122963 4186,4371,4465,4560,5671,6216,6670,7021,7140,7381,7503,7626,8385,8911,9045,10011 %N A122963 Triangular numbers with semiprime indices. %C A122963 Semiprime analog of A034953 Triangular numbers (A000217) with prime indices. %F A122963 a(n) = A000217(A001358(n)) = A001358(n)*(A001358(n)+1)/2. %e A122963 a(1) = semiprime(1)*(semiprime(1)+1)/2 = 4*5/2 = 10. %e A122963 a(2) = semiprime(2)*(semiprime(2)+1)/2 = 6*7/2 = 21. %e A122963 a(3) = semiprime(3)*(semiprime(3)+1)/2 = 9*10/2 = 45. %Y A122963 Cf. A000217, A001358, A034953. %K A122963 nonn %O A122963 1,1 %A A122963 _Jonathan Vos Post_, Oct 26 2006 %E A122963 a(41)-a(46) from _Giovanni Resta_, Jun 13 2016