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 A046411 Composite numbers the concatenation of whose prime factors is a prime. 12
 6, 12, 18, 21, 22, 28, 33, 39, 46, 51, 52, 54, 58, 63, 66, 70, 82, 84, 93, 98, 111, 115, 117, 133, 141, 142, 148, 154, 159, 162, 165, 166, 171, 172, 175, 177, 182, 187, 198, 201, 205, 207, 210, 219, 220, 226, 232, 235, 237, 245, 246, 247, 249, 253, 255, 261 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 COMMENTS For the corresponding primes, see A038514. - Lekraj Beedassy, Jun 05 2009 LINKS Charles R Greathouse IV, Table of n, a(n) for n = 1..10000 Patrick De Geest, Home Primes EXAMPLE 162 = 2 * 3 * 3 * 3 * 3 and 23333 is a prime, so 162 is in the sequence. MATHEMATICA co[n_, k_]:=Nest[FromDigits[Flatten[IntegerDigits[{#, n}]]]&, n, k-1]; Select[Range[261], !PrimeQ[#]&&PrimeQ[FromDigits[Flatten[IntegerDigits[co@@@FactorInteger[#]]]]]&](* Jayanta Basu, Jun 04 2013 *) PROG (PARI) is(n)=my(f=factor(n), s=""); for(i=1, #f~, for(j=1, f[i, 2], s=Str(s, f[i, 1]))); isprime(eval(s)) && !isprime(n) \\ Charles R Greathouse IV, May 14 2015 (Python) from sympy import isprime, factorint def ok(n): f = factorint(n) if sum(e for e in f.values()) < 2: return False return isprime(int("".join(str(p)*e for p, e in f.items()))) print(list(filter(ok, range(2, 262)))) # Michael S. Branicky, Jun 12 2021 CROSSREFS Cf. A037271-A037281. Cf. A038514 (corresponding primes), A221220 (factors without multiplicity). Sequence in context: A316221 A138939 A221220 * A364348 A364537 A350845 Adjacent sequences: A046408 A046409 A046410 * A046412 A046413 A046414 KEYWORD nonn,base AUTHOR Patrick De Geest, Jun 15 1998 EXTENSIONS Edited by Charles R Greathouse IV, Apr 23 2010 Title clarified by Sean A. Irvine, Jan 16 2021 STATUS approved

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Last modified February 29 17:14 EST 2024. Contains 370427 sequences. (Running on oeis4.)