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 A320584 #26 Dec 01 2018 09:36:45 %S A320584 2,3,5,7,20,21,22,23,24,25,26,27,28,29,30,31,32,33,34,35,36,37,38,39, %T A320584 50,51,52,53,54,55,56,57,58,59,70,71,72,73,74,75,76,77,78,79,200,201, %U A320584 202,203,204,205,206,207,208,209,210,211,212,213,214,215,216,217 %N A320584 Numbers whose first digit is prime. %t A320584 Select[Range@ 218, PrimeQ@ First@ IntegerDigits@ # &] (* _Michael De Vlieger_, Nov 05 2018 *) %o A320584 (PARI) isok(n) = isprime(digits(n)[1]); \\ _Michel Marcus_, Oct 17 2018 %o A320584 (PARI) a(n) = n--; my (w=1); while (n >= 4*w, n -= 4*w; w*=10); w*prime(1+n\w) + (n%w) \\ _Rémy Sigrist_, Nov 16 2018 %Y A320584 Supersequence of A320585 (primes whose first digit is prime). %Y A320584 Cf. A000030, A000040. %K A320584 nonn,base %O A320584 1,1 %A A320584 _Kritsada Moomuang_, Oct 16 2018