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A065588
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Smallest prime beginning with exactly n 5's.
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4
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2, 5, 557, 5557, 555521, 555557, 55555517, 55555553, 5555555501, 5555555557, 5555555555057, 555555555551, 5555555555551, 555555555555529, 555555555555557, 55555555555555519, 5555555555555555021, 555555555555555559, 55555555555555555567, 5555555555555555555087
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OFFSET
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0,1
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LINKS
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PROG
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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
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CROSSREFS
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KEYWORD
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nonn,base
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AUTHOR
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EXTENSIONS
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STATUS
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approved
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