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 A177688 #17 Sep 16 2018 02:11:22 %S A177688 0,1,4,6,7,9,12,13,15,18,19,22,25,28,31,33,39,46,48,49,52,60,61,64,67, %T A177688 73,75,84,85,88,90,99,100,103,106,132,133,135,136,138,142,156,160,163, %U A177688 171,178,181,183,187,190,198,201,202,211,220,222,229,238,241,246,252 %N A177688 Numbers n such that (n+2)//n - (n+1) is prime, where // represents the concatenation of decimals. %C A177688 If n is a k-digit number, then we demand that p = (n+2) * 10^k + n - (n+1) is a prime number, obviously of the form p = (n+2) * 10^k - 1, so the decimal representation of p is n+1 followed by k times the digit 9. %C A177688 The sequence is infinite, proof with Dirichlet's prime number (in arithmetic progressions) theorem. %C A177688 Note that numbers of the form (n+2)//n + (n+1) are multiples of 3 and do not generate primes. %e A177688 2//0 - 1 = 20 - 1 = 19 = prime(8), 0 is first term; %e A177688 3//1 - 2 = 31 - 2 = 29 = prime(10), 1 is 2nd term; %e A177688 6//4 - 5 = 64 - 5 = 59 = prime(17), 4 is 3rd term. %t A177688 n2ncQ[n_]:=PrimeQ[FromDigits[Join[IntegerDigits[n+2], IntegerDigits[ n]]]- n-1]; Select[Range[0,300],n2ncQ] (* _Harvey P. Dale_, Feb 24 2011 *) %Y A177688 Cf. A000040, A065582, A095995, A173976, A177435. %K A177688 base,easy,nonn %O A177688 1,3 %A A177688 Ulrich Krug (leuchtfeuer37(AT)gmx.de), May 11 2010