OFFSET
0,1
COMMENTS
For n <= 15392, a(n) = 0 if and only if n = 6930. Conjecture: if a(n) = 0, then n is divisible by 3. Conjecture verified for n <= 10^6. a(n) = 0 for n = 6930, 50358, 56574, 72975.
LINKS
Chai Wah Wu, Table of n, a(n) for n = 0..6068
Chai Wah Wu, On a conjecture regarding primality of numbers constructed from prepending and appending identical digits, arXiv:1503.08883 [math.NT], 2015.
PROG
(Python)
from gmpy2 import mpz, digits, is_prime
def A256481(n, limit=2000):
....if n in (6930, 50358, 56574, 72975):
........return 0
....if n == 0:
........return 2
....sn = str(n)
....for i in range(1, limit+1):
........for j in range(1, 10, 2):
............si = digits(j, 10)*i
............p = mpz(sn+si)
............if is_prime(p):
................return int(p)
....else:
........return 'search limit reached.'
CROSSREFS
KEYWORD
nonn,base
AUTHOR
Chai Wah Wu, Mar 31 2015
STATUS
approved