A266144 Number of n-digit primes in which n-1 of the digits are 5's.
4, 2, 1, 1, 0, 1, 0, 2, 0, 1, 0, 2, 1, 0, 1, 0, 0, 1, 0, 0, 0, 1, 0, 0, 0, 2, 0, 0, 0, 0, 0, 1, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 1, 0, 0, 0, 0, 0, 0, 0, 1, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 1, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0
Offset: 1
Examples
a(2) = 2 since 53 and 59 are primes. a(3) = 1 since 557 is the only prime.
Links
- Michael De Vlieger and Robert G. Wilson v, Table of n, a(n) for n = 1..1500
Crossrefs
Programs
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Mathematica
d = 5; Array[Length@ Select[d (10^# - 1)/9 + (Range[0, 9] - d), PrimeQ] &, 100]
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Python
from _future_ import division from sympy import isprime def A266144(n): return 4 if n==1 else sum(1 for d in [-4,-2,2,4] if isprime(5*(10**n-1)//9+d)) # Chai Wah Wu, Dec 27 2015
Comments