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 A108442 Number of paths from (0,0) to (3n,0) that stay in the first quadrant (but may touch the horizontal axis), consisting of steps u=(2,1), U=(1,2), or d=(1,-1) and having only u steps among the steps leading to the first d step. 3
 1, 1, 3, 15, 97, 721, 5827, 49759, 441729, 4035937, 37702723, 358474735, 3457592161, 33748593841, 332730216579, 3308635650495, 33145196426753, 334193815799233, 3388807714823043, 34537227997917391, 353578650475659617, 3634495706671023505, 37496621681376849219, 388135791657414454815 (list; graph; refs; listen; history; text; internal format)
 OFFSET 0,3 LINKS G. C. Greubel, Table of n, a(n) for n = 0..955 Emeric Deutsch, Problem 10658: Another Type of Lattice Path, American Math. Monthly, 107, 2000, 368-370. FORMULA G.f.: 1/(1-z*A), where A = 1 + z*A^2 + z*A^3 = (2/3)*sqrt((z+3)/z)*sin((1/3)*arcsin(sqrt(z)*(z+18)/(z+3)^(3/2)))-1/3 (the g.f. of A027307). a(n) = Sum_{k=1..n} (k*(Sum_{i=0..n-k} binomial(2*n-k, i)*binomial(3*n-2*k-i-1, 2*n-k-1))/(2*n-k)), n > 0, a(0)=1. - Vladimir Kruchinin, Oct 23 2011 G.f. y(x) satisfies: (3+x)*y*(1-y) + (1+x^2)*y^3 = 1. - Vaclav Kotesovec, Mar 17 2014 a(n) ~ (11+5*sqrt(5))^n / (5^(5/4) * sqrt(Pi) * n^(3/2) * 2^(n+1)). - Vaclav Kotesovec, Mar 17 2014 EXAMPLE a(2)=3 because we have udud, udUdd and uudd. MAPLE A:=(2/3)*sqrt((z+3)/z)*sin((1/3)*arcsin(sqrt(z)*(z+18)/(z+3)^(3/2)))-1/3: gser:=series(1/(1-z*A), z=0, 30): 1, seq(coeff(gser, z^n), n=1..25); MATHEMATICA Flatten[{1, Table[Sum[k*Sum[Binomial[2*n-k, i]*Binomial[3*n-2*k-i-1, 2*n-k-1], {i, 0, n-k}]/(2*n-k), {k, 1, n}], {n, 1, 20}]}] (* Vaclav Kotesovec, Mar 17 2014, after Vladimir Kruchinin *) PROG (Maxima) a(n):=if n=0 then 1 else sum((k*sum(binomial(2*n-k, i)*binomial(3*n-2*k-i-1, 2*n-k-1), i, 0, n-k))/(2*n-k), k, 1, n); /* Vladimir Kruchinin, Oct 23 2011 */ CROSSREFS Column 0 of A108441. Cf. A027307, A108441. Sequence in context: A079689 A262751 A231445 * A060148 A143435 A132437 Adjacent sequences:  A108439 A108440 A108441 * A108443 A108444 A108445 KEYWORD nonn AUTHOR Emeric Deutsch, Jun 08 2005 STATUS approved

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Last modified January 21 16:54 EST 2020. Contains 331114 sequences. (Running on oeis4.)