A281924 Least prime p such that p*n is a concatenation of two primes, -1 if it does not exist.

23, 11, 11, 13, 5, 317, 5, 29, 3, -1, 3, 11, 19, 293, 5, 7, 17, 239, 3, -1, 11, 241, 5, 3, 3, 227, 5, 19, 7, -1, 7, 41, 7, 853, 5, 17, 7, 29, 3, -1, 5, 31, 11, 3, 3, 37, 5, 29, 7, -1, 11, 61, 7, 13, 13, 47, 13, 19, 3, -1, 5, 821, 5, 3, 3, 47, 11, 29, 3, -1, 3, 11

Offset

1,1

Comments

If p*n = concat(a,b), leading 0 in b are admitted.

a(n) = -1 if a(n) mod 10 = 0.

Links

Paolo P. Lava, Table of n, a(n) for n = 1..2500

Example

a(1) = 23 because 23*1 = 23 = concat(2,3);
a(2) = 11 because 11*2 = 22 = concat(2,2);
a(6) = 317 because 317^6 = 1902 = concat(19,02).

Maple

with(numtheory): P:= proc(q) local a, k, n, ok;
for n from 1 to q do for a from 1 by 2 to q do if isprime(a) then ok:=0;
for k from 1 to ilog10(a*n) do if isprime(trunc(a*n/10^k)) and isprime(a*n mod 10^k) then ok:=1; break; fi; od; if ok=1 then print(a); break; fi; fi; od; od; end: P(10^9);

Crossrefs

Cf. A000040, A281923.

Sequence in context: A319046 A033343 A128364 * A054574 A363371 A016837

Adjacent sequences: A281921 A281922 A281923 * A281925 A281926 A281927

Keyword

sign,base,easy

Author

Paolo P. Lava, Feb 16 2017

Status

approved