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A337333
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Number of pairs of odd divisors of n, (d1,d2), such that d1 <= d2.
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2
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1, 1, 3, 1, 3, 3, 3, 1, 6, 3, 3, 3, 3, 3, 10, 1, 3, 6, 3, 3, 10, 3, 3, 3, 6, 3, 10, 3, 3, 10, 3, 1, 10, 3, 10, 6, 3, 3, 10, 3, 3, 10, 3, 3, 21, 3, 3, 3, 6, 6, 10, 3, 3, 10, 10, 3, 10, 3, 3, 10, 3, 3, 21, 1, 10, 10, 3, 3, 10, 10, 3, 6, 3, 3, 21, 3, 10, 10, 3, 3, 15, 3, 3, 10, 10
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OFFSET
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1,3
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COMMENTS
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Number of distinct rectangles that can be made whose side lengths are odd divisors of n.
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LINKS
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FORMULA
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a(n) = Sum_{d1|n, d2|n, d1 and d2 odd, d1<=d2} 1.
a(n) = 1 if and only if n = 2^k, k >= 0 (A000079).
a(n) = 3 if n is an odd prime. (End)
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EXAMPLE
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a(15) = 10; There are 10 pairs of odd divisors of 15, (d1,d2), such that d1<=d2. They are: (1,1), (1,3), (1,5), (1,15), (3,3), (3,5), (3,15), (5,5), (5,15), (15,15). So a(15) = 10.
a(16) = 1; (1,1) is the only pair of odd divisors of 16, (d1,d2), such that d1<=d2. So a(16) = 1.
a(17) = 3; There are 3 pairs of odd divisors of 17, (d1,d2), such that d1<=d2. They are (1,1), (1,17) and (17,17). So a(17) = 3.
a(18) = 6; There are 6 pairs of odd divisors of 18, (d1,d2), such that d1<=d2. They are: (1,1), (1,3), (1,9), (3,3), (3,9) and (9,9). So a(18) = 6.
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MATHEMATICA
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Table[Sum[Sum[Mod[i, 2]*Mod[k, 2] (1 - Ceiling[n/k] + Floor[n/k]) (1 - Ceiling[n/i] + Floor[n/i]), {i, k}], {k, n}], {n, 100}]
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PROG
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CROSSREFS
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KEYWORD
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nonn
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AUTHOR
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STATUS
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approved
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