A320584 Numbers whose first digit is prime.

2, 3, 5, 7, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 50, 51, 52, 53, 54, 55, 56, 57, 58, 59, 70, 71, 72, 73, 74, 75, 76, 77, 78, 79, 200, 201, 202, 203, 204, 205, 206, 207, 208, 209, 210, 211, 212, 213, 214, 215, 216, 217

Offset

1,1

Links

Table of n, a(n) for n=1..62.

Mathematica

Select[Range@ 218, PrimeQ@ First@ IntegerDigits@ # &] (* Michael De Vlieger, Nov 05 2018 *)

Prog

(PARI) isok(n) = isprime(digits(n)[1]); \\ Michel Marcus, Oct 17 2018
(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

Crossrefs

Supersequence of A320585 (primes whose first digit is prime).

Cf. A000030, A000040.

Sequence in context: A048420 A048405 A110457 * A209191 A162948 A024768

Adjacent sequences: A320581 A320582 A320583 * A320585 A320586 A320587

Keyword

nonn,base

Author

Kritsada Moomuang, Oct 16 2018

Status

approved