OFFSET
1,1
COMMENTS
The leading digits must be 4's and only the trailing digit can vary.
For n large a(n) is usually zero.
LINKS
Michael De Vlieger and Robert G. Wilson v, Table of n, a(n) for n = 1..1500
EXAMPLE
a(3) = 2 since 443 and 449 are primes.
a(4) = 2 since 4441 and 4447 are primes.
MATHEMATICA
d = 4; Array[Length@ Select[d (10^# - 1)/9 + (Range[0, 9] - d), PrimeQ] &, 100]
PROG
(Python)
from __future__ import division
from sympy import isprime
def A266143(n):
return 4 if n==1 else sum(1 for d in [-3, -1, 3, 5] if isprime(4*(10**n-1)//9+d)) # Chai Wah Wu, Dec 27 2015
CROSSREFS
KEYWORD
nonn,base
AUTHOR
Michael De Vlieger and Robert G. Wilson v, Dec 21 2015
STATUS
approved