A253427 Number of left-truncatable n-digit primes with least significant digit 7.

1, 5, 17, 47, 89, 150, 199, 238, 264, 257, 230, 181, 139, 117, 70, 46, 33, 19, 11, 5, 5, 4, 3, 1

Offset

1,2

Comments

Number of n-digit (n <= 24) truncatable primes (i.e., left-truncatable with the least significant digit 7; only prime suffixes without digit '0'). Formed by successively prepending a digit (1,...,9) to the most significant digit (1st is 7) while preserving primality; a(13) with 139 members includes A253386 (includes the palindromic prime suffix 7693967), but a(24) includes just a single one, 357686312646216567629137 (contains the nonprime palindrome 1264621).

Links

Table of n, a(n) for n=1..24.

I. O. Angell and H. J. Godwin, On Truncatable Primes, Math. Comput. 31, 265-267, 1977.

Eric Weisstein's World of Mathematics, Truncatable Prime

Wikipedia, Truncatable prime

Index entries for sequences related to truncatable primes

Example

a(1)=1 {7};
a(2)=5 {17,37,47,67,97};
...
a(23)=3 {57686312646216567629137, 95918918997653319693967, 96686312646216567629137};
a(24)=1 {357686312646216567629137}; see also A012885.

Maple

S[1]:= {7}:
for n from 2 while nops(S[n-1]) > 0 do
  S[n]:= select(isprime, {seq(seq(i*10^(n-1)+s, i=1..9), s = S[n-1])})
od:
seq(nops(S[i]), i=1..n-2); # Robert Israel, Jan 01 2015

Crossrefs

Cf. A012885, A024785, A050987, A253386.

Sequence in context: A046787 A003295 A228857 * A339805 A338972 A332358

Adjacent sequences: A253424 A253425 A253426 * A253428 A253429 A253430

Keyword

base,easy,fini,full,nonn

Author

Mikk Heidemaa, Dec 31 2014

Status

approved