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A002557
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Odd squarefree numbers with an even number of prime factors that have no prime factors greater than 31.
(Formerly M4959 N2126)
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3
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1, 15, 21, 33, 35, 39, 51, 55, 57, 65, 69, 77, 85, 87, 91, 93, 95, 115, 119, 133, 143, 145, 155, 161, 187, 203, 209, 217, 221, 247, 253, 299, 319, 323, 341, 377, 391, 403, 437, 493, 527, 551, 589, 667, 713, 899, 1155, 1365, 1785, 1995, 2145, 2415, 2805, 3003
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OFFSET
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1,2
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COMMENTS
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Original name: A subset of A056913, definition unclear.
The definition is given on page 70 of Gupta (1943), but is hard to understand.
The b-file contains the full sequence. - Robert Israel, Jan 21 2016
The sequence is closed under the commutative binary operation A059897(.,.). As integers are self-inverse under A059897, it forms a subgroup of the positive integers considered as a group under A059897. A subgroup of A056913. - Peter Munn, Jan 16 2020
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REFERENCES
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H. Gupta, A formula for L(n), J. Indian Math. Soc., 7 (1943), 68-71.
N. J. A. Sloane, A Handbook of Integer Sequences, Academic Press, 1973 (includes this sequence).
N. J. A. Sloane and Simon Plouffe, The Encyclopedia of Integer Sequences, Academic Press, 1995 (includes this sequence).
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LINKS
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H. Gupta, A formula for L(n), J. Indian Math. Soc., 7 (1943), 68-71. [Annotated scanned copy]
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MAPLE
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S:= select(t -> (nops(t)::even), combinat:-powerset(select(isprime, [seq(i, i=3..31, 2)]))):
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PROG
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(Magma) a:= func< n | Factorization(n)>; [1] cat [n: n in [3..3003 by 2] | IsSquarefree(n) and (-1)^&+[p[2]: p in a(n)] eq 1 and f[#f][1] le 31 where f is a(n)]; // Arkadiusz Wesolowski, Jan 21 2016
(Python) powerset = lambda lst: reduce(lambda result, x: result + [subset + [x] for subset in result], lst, [[]])
product = lambda lst: reduce(lambda x, y: x*y, lst, 1)
primes = [3, 5, 7, 11, 13, 17, 19, 23, 29, 31]
sequence = sorted(product(s) for s in powerset(primes) if len(s) % 2 == 0) # David Radcliffe, Jan 21 2016
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CROSSREFS
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KEYWORD
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nonn,full,fini
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AUTHOR
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EXTENSIONS
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STATUS
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approved
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