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A342352
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Expansion of e.g.f. (exp(x)-1)*(exp(x) - x^2/2 - x - 1).
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2
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0, 0, 0, 0, 4, 15, 41, 98, 218, 465, 967, 1980, 4016, 8099, 16277, 32646, 65398, 130917, 261971, 524096, 1048364, 2096919, 4194049, 8388330, 16776914, 33554105, 67108511, 134217348, 268435048, 536870475, 1073741357, 2147483150, 4294966766, 8589934029
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OFFSET
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0,5
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COMMENTS
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a(n) is the number of binary strings of length n that contain at least three 0's but not all digits are 0.
a(n) is also the number of proper subsets with at least three elements of an n-element set.
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LINKS
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FORMULA
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a(n) = 2^n - Sum_{i={0,1,2,n}} binomial(n,i).
G.f.: x^4*(2*x^2-5*x+4)/((2*x-1)*(x-1)^3). - Alois P. Heinz, Mar 09 2021
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EXAMPLE
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a(6) = 41 since the strings are the 20 permutations of 000111, the 15 permutations of 000011, and the 6 permutations of 000001.
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CROSSREFS
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KEYWORD
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nonn,easy
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AUTHOR
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STATUS
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approved
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