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 A121898 #5 Dec 15 2017 17:37:06 %S A121898 120,300,528,780,2628,3240,3828,5460,13530,18528,19110,22578,25878, %T A121898 31878,32640,37128,49770,56280,64980,72390,73920,78210,103740,105570, %U A121898 115440,137550,159330,161028,277140,288420,316410,335790,370230,386760 %N A121898 Triangular numbers that are sandwiched between two semiprimes; or triangular numbers t such that t-1 and t+1 are both semiprime. %C A121898 Why are so many of the t-1's brilliant numbers (A078972)? %t A121898 Select[Accumulate[Range[1000]],PrimeOmega[#-1]==PrimeOmega[#+1]==2&] (* _Harvey P. Dale_, Jul 06 2014 *) %K A121898 nonn %O A121898 1,1 %A A121898 _Jason Earls_, Sep 01 2006