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

%I A253427 #75 Feb 16 2025 08:33:24
%S A253427 1,5,17,47,89,150,199,238,264,257,230,181,139,117,70,46,33,19,11,5,5,
%T A253427 4,3,1
%N A253427 Number of left-truncatable n-digit primes with least significant digit 7.
%C A253427 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).
%H A253427 I. O. Angell and H. J. Godwin, <a href="http://dx.doi.org/10.1090/S0025-5718-1977-0427213-2">On Truncatable Primes</a>, Math. Comput. 31, 265-267, 1977.
%H A253427 Eric Weisstein's World of Mathematics, <a href="https://mathworld.wolfram.com/TruncatablePrime.html">Truncatable Prime</a>
%H A253427 Wikipedia, <a href="http://en.wikipedia.org/wiki/Truncatable_prime">Truncatable prime</a>
%H A253427 <a href="/index/Tri#tprime">Index entries for sequences related to truncatable primes</a>
%e A253427 a(1)=1 {7};
%e A253427 a(2)=5 {17,37,47,67,97};
%e A253427 ...
%e A253427 a(23)=3 {57686312646216567629137, 95918918997653319693967, 96686312646216567629137};
%e A253427 a(24)=1 {357686312646216567629137}; see also A012885.
%p A253427 S[1]:= {7}:
%p A253427 for n from 2 while nops(S[n-1]) > 0 do
%p A253427   S[n]:= select(isprime, {seq(seq(i*10^(n-1)+s, i=1..9),s = S[n-1])})
%p A253427 od:
%p A253427 seq(nops(S[i]),i=1..n-2); # _Robert Israel_, Jan 01 2015
%Y A253427 Cf. A012885, A024785, A050987, A253386.
%K A253427 base,easy,fini,full,nonn
%O A253427 1,2
%A A253427 _Mikk Heidemaa_, Dec 31 2014