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 A221220 Numbers with more than one prime factor such that concatenation of its prime factors (without multiplicity) is a prime. 1
 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 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 LINKS Table of n, a(n) for n=1..57. EXAMPLE Prime factors of 140 are 2, 5, and 7 and 257 is prime, so 140 is a term. MATHEMATICA Select[Range[220], Length[x=First/@FactorInteger[#]]>1&&PrimeQ[FromDigits[Flatten[IntegerDigits[x]]]]&] PROG (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))) print(list(filter(ok, range(2, 220)))) # Michael S. Branicky, Jun 12 2021 CROSSREFS Cf. A024619, A046411. Sequence in context: A348719 A316221 A138939 * A046411 A364348 A364537 Adjacent sequences: A221217 A221218 A221219 * A221221 A221222 A221223 KEYWORD nonn,base,less AUTHOR Jayanta Basu, Jun 04 2013 STATUS approved

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Last modified February 29 13:41 EST 2024. Contains 370425 sequences. (Running on oeis4.)