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 A215647 #10 Aug 21 2012 00:45:21 %S A215647 4,415,410111,4101061003,410106100310003,410106100310001100001, %T A215647 4101061003100011000011000057,410106100310001100001100000110000051, %U A215647 410106100310001100001100000110000001100000001 %N A215647 Smallest semiprime whose decimal expansion consists of the concatenation of a 1-digit semiprime, a 2-digit semiprime, a 3-digit semiprime, ..., and an n-digit semiprime. %C A215647 This is to A215641 as semiprimes A001358 are to primes A000040. It is a plausible conjecture that a(n) always exists. %C A215647 If a(n) exists it has A000217(n) = n(n+1)/2 digits. %e A215647 a(2) = 415 = 5 * 83 = concatenation of 4 and 15, where 4 is the semiprime 2^2 and 15 is the semiprime 3 * 5. %e A215647 a(3) = 410111 = 13 * 31547 = the concatenation of 4, 10, and 111 where 4 = 2^2, 10 = 2 * 5, and 111 = 3 * 37. %e A215647 a(5) = 410106100310003 = 76871 * 5334991093 = Concatenate(4,10,106,1003,10003). %Y A215647 Cf. A001358, A215641. %K A215647 nonn,base %O A215647 1,1 %A A215647 _Jonathan Vos Post_, Aug 19 2012