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A221220
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Numbers with more than one prime factor such that concatenation of its prime factors (without multiplicity) is a prime.
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1
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6, 12, 18, 21, 22, 24, 33, 36, 39, 44, 46, 48, 51, 54, 58, 63, 66, 70, 72, 82, 88, 92, 93, 96, 99, 108, 111, 115, 116, 117, 132, 133, 140, 141, 142, 144, 147, 153, 154, 159, 162, 164, 165, 166, 176, 177, 182, 184, 187, 189, 192, 198, 201, 205, 210, 216, 219
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listen;
history;
text;
internal format)
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OFFSET
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1,1
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LINKS
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EXAMPLE
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Prime factors of 140 are 2, 5, and 7 and 257 is prime, so 140 is a term.
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MATHEMATICA
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Select[Range[220], Length[x=First/@FactorInteger[#]]>1&&PrimeQ[FromDigits[Flatten[IntegerDigits[x]]]]&]
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PROG
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(Python)
from sympy import isprime, primefactors
def ok(n):
pf = primefactors(n)
if len(pf) < 2: return False
return isprime(int("".join(str(p) for p in pf)))
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CROSSREFS
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KEYWORD
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nonn,base,less
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AUTHOR
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STATUS
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approved
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