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Search: id:A123611
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| 1, 2, 4, 8, 20, 52, 160, 492, 1620, 5408, 18504, 64132, 225440, 800048, 2865720, 10341208, 37568340, 137270956, 504176992, 1860277044, 6892335720, 25631327688, 95640894056, 357975249028, 1343650267296, 5056424257552
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OFFSET
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0,2
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FORMULA
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a(n) = 2*A047996(2*n,n) = 2*A003239(n) for n>0, where A047996 is the triangle of circular binomial coefficients and A003239(n) = number of rooted planar trees with n non-root nodes.
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PROGRAM
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(PARI) {a(n)=sum(k=0, n, if(k==0, 1, (1/n)*sumdiv(n, d, if(gcd(k, d)==d, eulerphi(d)*binomial(n/d, k/d)^2, 0))))}
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CROSSREFS
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Cf. A047996, A003239; A123610 (triangle), A123612 (antidiagonal sums); central terms: A123617, A123618.
Sequence in context: A078160 A089976 A000980 this_sequence A082279 A113180 A000116
Adjacent sequences: A123608 A123609 A123610 this_sequence A123612 A123613 A123614
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KEYWORD
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nonn
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AUTHOR
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Paul D. Hanna (pauldhanna(AT)juno.com), Oct 03 2006
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