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 A261070 Irregular triangle read by rows: T(n,k) is the number of arrangements of n circles with 2k intersections (using the same rules as A250001). 6
 1, 1, 2, 1, 4, 4, 2, 4, 9, 15 (list; graph; refs; listen; history; text; internal format)
 OFFSET 0,3 COMMENTS Length of n-th row: 1 + (n-1)n/2 (for a configuration for T(n,(n-1)n/2), consider n circles of radius 1 and centers at (k/n,0) for 1<=k<=n). The generating function down the column k=1 is 1+z^2 *C^2(z) *[C^2(z)+C(z^2)]/ (2*[1-z*C(z)]) = 1+ z^2 +4*z^3 +15*z^4+ 50*z^5+...where C(z) = 1+z+2*z^2+4*z^3+... is the g.f. of A000081 divided by z; eq. (78) in arXiv:1603.00077. - R. J. Mathar, Mar 05 2016 LINKS FORMULA A250001(n) = Sum_{k>=0} T(n,k). A000081(n+1) = T(n,0). EXAMPLE n\k 0  1  2  3  4  5  6 0   1 1   1 2   2  1 3   4  4  2  4 4   9 15  .  .  .  .  . 5  20  . CROSSREFS Row sums give A250001. Cf. A000081, A249752, A252158, A280786 (column k=1) Sequence in context: A128250 A086145 A309086 * A249140 A113421 A135366 Adjacent sequences:  A261067 A261068 A261069 * A261071 A261072 A261073 KEYWORD nonn,more,tabf AUTHOR Benoit Jubin, Aug 08 2015 STATUS approved

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Last modified April 3 04:21 EDT 2020. Contains 333195 sequences. (Running on oeis4.)