A065588 Smallest prime beginning with exactly n 5's.
2, 5, 557, 5557, 555521, 555557, 55555517, 55555553, 5555555501, 5555555557, 5555555555057, 555555555551, 5555555555551, 555555555555529, 555555555555557, 55555555555555519, 5555555555555555021, 555555555555555559, 55555555555555555567, 5555555555555555555087
Offset: 0
Links
- Michael S. Branicky, Table of n, a(n) for n = 0..997 (terms 0..200 from M. F. Hasler)
Programs
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Python
from sympy import isprime def a(n): if n < 2: return list([2, 5])[n] n5s, i, pow10, end_digits = int('5'*n), 1, 1, 0 while True: i = 1 while i < pow10: istr = str(i) if istr[0] == '5' and len(istr) == end_digits: i += 2 if pow10 <= 10 else pow10 // 10 else: t = n5s * pow10 + i if isprime(t): return t i += 2 pow10 *= 10; end_digits += 1 print([a(n) for n in range(20)]) # Michael S. Branicky, Feb 05 2021, corrected Mar 03 2021
Extensions
Corrected by N. J. A. Sloane, Jan 11 2007 and by Don Reble, Jan 17 2007
Offset corrected by Michael S. Branicky, Feb 05 2021