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 A215685 #12 Aug 23 2012 19:57:04 %S A215685 13,13337,131071021,13107100910711,13107100910007100483, %T A215685 131071009100071000491000187,13107100910007100049100003310000657, %U A215685 13107100910007100049100003310000169100007543,131071009100071000491000033100001691000000071000015351 %N A215685 Smallest prime whose decimal expansion consists of the concatenation of a 2-digit emirp, a 3-digit emirp, a 4-digit emirp, ..., and an n-digit emirp (A006567). %C A215685 If a(n) exists it has A000217(n)-1 digits. %e A215685 a(2) = 13 which is a prime, and whose decimal digits reversed, 31, is also a prime. %e A215685 a(3) = 13337, which is a prime, and the concatenation of 13 (an emirp) and 337 (an emirp because 733 is also a prime). It happens that the digital reversal of a(3), 73331, is also prime, so that 13337 is an emirp, but that is not a requirement for this sequence. %e A215685 Note that a(4) is a prime but not an emirp, because 12070131 = 3 * 61 * 65957. %Y A215685 Cf. A000040, A006567, A215641, A215647. %K A215685 nonn,base,easy %O A215685 2,1 %A A215685 _Jonathan Vos Post_, Aug 20 2012 %E A215685 More terms from _Alois P. Heinz_, Aug 22 2012