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A264815
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Semirps: a semirp (or semi-r-p) is a semiprime r*p with r and p both reversed primes.
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1
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4, 6, 9, 10, 14, 15, 21, 22, 25, 26, 33, 34, 35, 39, 49, 51, 55, 62, 65, 74, 77, 85, 91, 93, 111, 119, 121, 142, 143, 146, 155, 158, 169, 185, 187, 194, 202, 213, 214, 217, 219, 221, 226, 237, 259, 262, 289, 291, 298, 302, 303, 314, 321, 334, 339, 341, 355
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OFFSET
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1,1
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COMMENTS
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A semiprime (A001358) is the product of two prime, not necessarily distinct. A semiprime is in this list if those two primes (A000040) are reversed primes (A004087).
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LINKS
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FORMULA
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EXAMPLE
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9 is in the list because 9 = 3*3 is a semiprime and reverse(3) = 3 is prime.
143 is in the list because 143 = 11*13 is a semiprime and both reverse(11) = 11 and reverse(13) = 31 are prime.
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PROG
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(Sage)
reverse = lambda n: sum([10^i*int(str(n)[i]) for i in range(len(str(n)))])
def is_semirp(n):
F = factor(n)
if sum([f[1] for f in F])==2:
r, p = F[0][0], F[-1][0]
if is_prime(reverse(r)) and is_prime(reverse(p)): return True
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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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STATUS
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approved
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