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A155069
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Expansion of (3-x-sqrt(1-6*x+x^2))/2.
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4
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1, 1, 2, 6, 22, 90, 394, 1806, 8558, 41586, 206098, 1037718, 5293446, 27297738, 142078746, 745387038, 3937603038, 20927156706, 111818026018, 600318853926, 3236724317174, 17518619320890, 95149655201962, 518431875418926
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OFFSET
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0,3
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COMMENTS
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A minor variation of A006318. Unsigned version of A086456 and A103137. The Hankel transform of this sequence is A006125.
a(n) is also the number of "branching configurations" for RNA (see Sankoff, 1985) that have exactly n hairpins. [From Lee A. Newberg, Mar 30 2010]
a(n) is also the number of ways to insert balanced parentheses into a product of n variables such that each parenthesis pair has 2 or more top-level factors. [From Lee A. Newberg, Apr 06 2010]
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REFERENCES
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S. Kitaev, Patterns in Permutations and Words, Springer-Verlag, 2011. see p. 399 Table A.7
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LINKS
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Table of n, a(n) for n=0..23.
Sankoff (1985) Simultaneous solution of the RNA folding, alignment and protosequence problems, Siam J. Appl. Math 45(5):810-825. [From Lee A. Newberg, Mar 30 2010]
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FORMULA
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G.f.: (3-x-sqrt(1-6x+x^2))/2.
G.f.: 4 / (3 - x + sqrt(1 - 6*x + x^2)). - Michael Somos, Apr 18 2012
a(n) ~ sqrt((sqrt(18)-4)/(4*Pi)) * n^(-3/2) * (3 + sqrt(8))^n, which is, approximately, a(n) ~ 0.1389558645 * n^(-1.5) * 5.828427099^n. [From Lee A. Newberg, Apr 06 2010]
a(n) = top left term of M^n, where M = the production matrix:
1, 1, 0, 0, 0, ...
1, 2, 1, 0, 0, ...
1, 2, 2, 1, 0, ...
1, 2, 2, 2, 1, ...
1, 2, 2, 2, 2, 1, ...
...
Top row terms of M^n generates rows of triangle A132372. - Gary W. Adamson, Jul 07 2011
G.f.: A(x)=(3-x-sqrt(1-6x+x^2))/2= 2 - G(0); G(k)= 1 + x - 2*x/G(k+1); (continued fraction ,1-step, 1var.). - Sergei N. Gladkovskii, Jan 04 2012
G.f.: A(x)=(3-x-sqrt(1-6x+x^2))/2= G(0); G(k)= := 1 - x/(1 - 2/G(k+1)); (continued fraction ,2-step, 2var.). - Sergei N. Gladkovskii, Jan 04 2012
Conjecture: n*a(n) +3*(3-2*n)*a(n-1) +(n-3)*a(n-2)=0. - R. J. Mathar, Jul 24 2012
G.f.: 1 / (1 - x / (1 - x / (1 - 2*x / (1 - x / (1 - 2*x / (1 - x / ... )))))) = 1 + x / (1 - 2*x / (1 - x / (1 - 2*x / (1 - x / (1 - 2*x / (1 - x / ... )))))). - Michael Somos, Jan 03 2013
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EXAMPLE
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Contribution from Lee A. Newberg, Mar 30 2010: (Start)
For n = 2, the a(2) = 2 branching configurations are ()() and (()()), where each () indicates a hairpin (also termed 1-loop) and each other pair of parentheses indicates a k-loop for k >= 3.
For n = 3, the a(3) = 6 branching configurations are ()()(), (()())(), ()(()()), (()()()), ((()())()), and (()(()())). (End)
When inserting balanced parentheses into the product x^n: For n = 0, the a(0) = 1 possible term is the empty term. For n = 1, the a(1) = 1 possible term is x. For n = 2, the a(2) = 2 possible terms are xx and (xx). For n = 3, the a(3) = 6 possible terms are xxx, (xx)x, x(xx), (xxx), ((xx)x), and (x(xx)). [From Lee A. Newberg, Apr 06 2010]
1 + x + 2*x^2 + 6*x^3 + 22*x^4 + 90*x^5 + 394*x^6 + 1806*x^7 + ...
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CROSSREFS
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Cf. A006318, A085403, A086456, A103137.
Cf. A132372
Sequence in context: A049134 A086456 A006318 * A103137 A165546 A053617
Adjacent sequences: A155066 A155067 A155068 * A155070 A155071 A155072
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KEYWORD
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nonn
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AUTHOR
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Philippe DELEHAM, Nov 02 2009
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STATUS
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approved
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