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A065205
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Number of subsets of proper divisors of n that sum to n.
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18
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0, 0, 0, 0, 0, 1, 0, 0, 0, 0, 0, 2, 0, 0, 0, 0, 0, 2, 0, 1, 0, 0, 0, 5, 0, 0, 0, 1, 0, 3, 0, 0, 0, 0, 0, 7, 0, 0, 0, 3, 0, 2, 0, 0, 0, 0, 0, 10, 0, 0, 0, 0, 0, 3, 0, 2, 0, 0, 0, 34, 0, 0, 0, 0, 0, 2, 0, 0, 0, 0, 0, 31, 0, 0, 0, 0, 0, 1, 0, 6, 0, 0, 0, 25, 0, 0, 0, 1, 0, 23, 0, 0, 0, 0, 0, 21, 0, 0, 0, 2
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OFFSET
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1,12
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COMMENTS
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Deficient and weird numbers have a(n) = 0, perfect numbers and others (see A064771) have a(n) = 1.
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LINKS
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FORMULA
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EXAMPLE
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a(20) = 1 because {1, 4, 5, 10} is the only subset of proper divisors of 20 that sum to 20.
a(24) = 5 because there are five different subsets we can use to sum up to 24: {1, 2, 3, 4, 6, 8}, {1, 2, 3, 6, 12}, {1, 3, 8, 12}, {2, 4, 6, 12}, {4, 8, 12}.
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MATHEMATICA
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a[n_] := (dd = Most[ Divisors[n] ]; cc = Array[c, Length[dd]]; Length[ {ToRules[ Reduce[ And @@ (0 <= # <= 1 &) /@ cc && dd . cc == n, cc, Integers]]}]); Table[ a[n], {n, 1, 100}] (* Jean-François Alcover, Feb 23 2012 *)
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PROG
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(Haskell)
a065205 n = p (a027751_row n) n where
p _ 0 = 1
p [] _ = 0
p (k:ks) m = if m < k then 0 else p ks (m - k) + p ks m
(PARI) a(n, s, d)={s || (s=sigma(n)-n) || return; d||d=vecextract(divisors(n), "^-1"); while(d[#d]>n, s-=d[#d]; d=d[1..-2]); s<=n && return(s==n); if( n>d[#d], a(n-d[#d], s-d[#d], d[1..-2]), 1)+a(n, s-d[#d], d[1..-2])} \\ M. F. Hasler, May 11 2015
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CROSSREFS
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KEYWORD
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nonn
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AUTHOR
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Jonathan Ayres (jonathan.ayres(AT)btinternet.com), Oct 19 2001
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EXTENSIONS
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More terms and additional comments from Jud McCranie, Oct 21 2001
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STATUS
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approved
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