A068105 Smallest prime starting with n 5s.

2, 5, 557, 5557, 555521, 555557, 55555517, 55555553, 5555555501, 5555555557, 555555555551, 555555555551, 5555555555551, 555555555555529, 555555555555557, 55555555555555519, 555555555555555559, 555555555555555559, 55555555555555555567, 5555555555555555555087

Offset: 0

Author

Amarnath Murthy, Feb 20 2002

Keywords

• base
• easy
• nonn

Links

• Michael S. Branicky, Table of n, a(n) for n = 0..1000

Crossrefs

Cf. A065584, A068103, A068104, A065587, A065588.

Programs

Python

from sympy import isprime
def a(n):
  if n < 2: return list([2, 5])[n]
  n5s, i, pow10 = int('5'*n), 1, 1
  while True:
    i = 1
    while i < pow10:
      t = n5s * pow10 + i
      if isprime(t): return t
      i += 2
    pow10 *= 10
print([a(n) for n in range(20)]) # Michael S. Branicky, Feb 05 2021

Formula

a(n) <= A065588(n). - Michael S. Branicky, Feb 05 2021

Extensions

Edited and extended by Robert G. Wilson v, Feb 21 2002

Corrected by Don Reble, Jan 17 2007