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
KEYWORD
sign,base,easy
AUTHOR
Paolo P. Lava, Feb 16 2017
STATUS
approved