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 A176781 #3 Mar 30 2012 17:40:23 %S A176781 3,11,3,17,3,3,3,11,89,41,257,3,29,131,353,3,3,11,89,521,257,3,17,3, %T A176781 467,89,149,17,71,47,293,17,191,47,3,41,23,11,401,41,443,41,293,479, %U A176781 311,23,587,41,1289,1013,29,41,59,293,1031,17,23,17,347,401,599,11,227,827,401 %N A176781 Smallest prime prime(i) such that concatenation 2//0_(n)//prime(i) is prime. %C A176781 We search for the prime such that the first prime (=2) concatenated with n zeros and concatenated with that prime is again a prime number. %C A176781 If p = prime(i) is a d(i)-digit prime: q = 2 * 10^(n+d(i)) + p has to be prime. %C A176781 Necessarily prime(i) is congruent to 2 (mod 3). %C A176781 It is conjectured that prime(i) = 3 occurs infinitely often: at n= 0, 2, 4, 5, 6, 11, 15, 16, 21, 23, 34, 114, 119,... %D A176781 E. I. Ignatjew, Mathematische Spielereien, Urania Verlag Leipzig/Jena/ Berlin 1982 %e A176781 n = 0: 2//3 = 23 = prime(9), 3 = prime(2) is first term %e A176781 n = 1: 2//0//11 = 2011 = prime(305), 11 = prime(5) is 2nd term %e A176781 n = 2: 2//00//3 = 2003 = prime(304), 3 = prime(2) is 3rd term %Y A176781 Cf. A164968, A173291, A176316 %K A176781 base,nonn %O A176781 0,1 %A A176781 Eva-Maria Zschorn (e-m.zschorn(AT)zaschendorf.km3.de), Apr 26 2010 %E A176781 Offset corrected and sequence extended by _R. J. Mathar_, Apr 28 2010