OFFSET
0,2
COMMENTS
a(n) = 0 if n is even or a multiple of 5. Conjecture: all other terms are nonzero. Conjecture verified for n <= 10^7.
"Appending" means "on the right".
LINKS
Chai Wah Wu, Table of n, a(n) for n = 0..10000
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 digits, mpz, is_prime
def A256480(n, limit=2000):
....sn = str(n)
....if not (n % 2 and n % 5):
........return 0
....for i in range(1, limit+1):
........for j in range(1, 10):
............si = digits(j, 10)*i
............p = mpz(si+sn)
............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