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A182721
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Mountain emirps.
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2
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1231, 1321, 1381, 1471, 1741, 1831, 12491, 12641, 12841, 13591, 13751, 13781, 13841, 14591, 14621, 14821, 14831, 14891, 15731, 15791, 18731, 19421, 19531, 19541, 19751, 19841, 123731, 123821, 124951, 124981, 125641, 125651, 125791, 125821, 125941, 126761, 126851
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OFFSET
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1,1
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COMMENTS
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The smallest mountain emirp 1231 and other terms of this sequence was mentioned by Loungrides in Prime Curios! (see link).
Question: How many are there?
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LINKS
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FORMULA
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EXAMPLE
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Illustration of a(11) = 13751 as a mountain emirp:
. . . . .
. . . . .
. . 7 . .
. . . . .
. . . 5 .
. . . . .
. 3 . . .
. . . . .
1 . . . 1
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PROG
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from sympy import isprime
def is_emirp(n):
if not isprime(n): return False
revn = int(str(n)[::-1])
return n != revn and isprime(revn)
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CROSSREFS
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KEYWORD
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nonn,base,fini,full
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AUTHOR
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EXTENSIONS
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STATUS
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approved
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