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A215974
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Numbers n such that Sum_{k=1..n} k!/2^k is an integer.
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5
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0, 2, 5, 12, 14, 25, 29, 54, 60, 62, 3445, 108995, 3625182, 13951972, 28010901, 7165572247, 14335792539, 114636743486, 229264368709, 458534096494
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OFFSET
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1,2
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COMMENTS
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This sequence lists the indices n for which A215976(n)=0 (power of 2 in denominator) and for which A215990 (numerator of the sum) may be even.
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LINKS
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FORMULA
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A215974(n) = A215972(n)-1 for all n. (The two sequences differ only in the use of the upper limit. The present convention seems more natural, the other one was used in the post on the NmbrThry list.)
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EXAMPLE
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a(1)=0 is in the sequence because sum(..., 1 <= k <= 0)=0 (empty sum) is an integer.
1 is not in the sequence because 1!/2^1 = 1/2 is not an integer.
a(2)=2 is in the sequence because 1!/2^1 + 2!/2^2 = 1 is an integer.
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MATHEMATICA
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sum = 0; Select[Range[0, 10^4], IntegerQ[sum += #!/2^#] &] (* Robert Price, Apr 04 2019 *)
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PROG
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(PARI) is_A215974(n)=denominator(sum(k=1, n, k!/2^k))==1
(PARI) s=0; for(k=1, 9e9, denominator(s+=k!/2^k)==1&print1(k, ", "))
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CROSSREFS
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KEYWORD
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nonn,more
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AUTHOR
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EXTENSIONS
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Terms through a(20) from Aart Blokhuis and Benne de Weger, Aug 30 2012, who thank Jan Willem Knopper for efficient programming. - N. J. A. Sloane, Aug 30 2012
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STATUS
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approved
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