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 A175251 #2 Mar 30 2012 19:00:23 %S A175251 4,9,12,15,16,18,21,24,25,26,28,32,33,34,36,38,39,40,42,45,48,49,50, %T A175251 51,52,55,56,57,60,63,64,65,68,69,70,72,74,76,77,78,80,81,84,86,87,88, %U A175251 90,91,93,94,95,98,100 %N A175251 Composites (A002808) with nonprime (A018252) subscripts. %C A175251 a(n) = composite(nonprime(n)) = A002808(A018252(n)). a(n) U A065858(n) = A002808(n), a(n+1) U A022449(n) = A002808(n) for n >= 1. a(1) = 4, a(n) = A050435(n-1) = composites (A002808) with composite (A002808) subscripts for n >=2. %e A175251 a(5) = 16 because a(5) = c(b(5)) = c(9) = 16, c = composite, b = nonprime. %K A175251 nonn %O A175251 1,1 %A A175251 _Jaroslav Krizek_, Mar 13 2010