A093156 Another approximation to A000084(n).

1, 2, 4, 9, 24, 63, 177, 514, 1527, 4625, 14230, 44358, 139779, 444558, 1425151, 4600339, 14939849, 48778197, 160019885, 527200711, 1743607826, 5786756470, 19266336882, 64331275266, 215377539119, 722840378042, 2431459847364

Offset

1,2

Comments

A more accurate version of A001573.

References

J. Riordan and C. E. Shannon, The number of two-terminal series-parallel networks, J. Math. Phys., 21 (1942), 83-93. Reprinted in Claude Elwood Shannon: Collected Papers, edited by N. J. A. Sloane and A. D. Wyner, IEEE Press, NY, 1993, pp. 560-570.

Links

Table of n, a(n) for n=1..27.

Formula

a(n) is the nearest integer to u(n) (with round(0.5)=0), where u(n) has o.g.f.: U(x) = (5 - 3*x - 2*x^2 - sqrt(9 - 30*x - 11*x^2 + 12*x^3 + 4*x^4))/2. [Riordan & Shannon]

Crossrefs

Cf. A000084, A058585.

Sequence in context: A261367 A148077 A148078 * A148079 A148080 A148081

Adjacent sequences: A093153 A093154 A093155 * A093157 A093158 A093159

Keyword

nonn

Author

Pab Ter (pabrlos(AT)yahoo.com), May 11 2004

Status

approved