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 A168597 Squares of the central trinomial coefficients (A002426). 5
 1, 1, 9, 49, 361, 2601, 19881, 154449, 1225449, 9853321, 80156209, 658076409, 5444816521, 45343869481, 379735715529, 3195538786449, 27004932177129, 229066136374761, 1949470542590481, 16640188083903609, 142415188146838161 (list; graph; refs; listen; history; text; internal format)
 OFFSET 0,3 COMMENTS Ignoring initial term, a(n) equals the logarithmic derivative of A168598. Partial sums of A007987. Hence, a(n) is the number of irreducible words of length at most 2n in the free group with generators x,y such that the total degree of x and the total degree of y both equal zero. - Max Alekseyev, Jun 05 2011 The number of ways a king, starting at the origin of an infinite chessboard, can return to the origin in n moves, where leaving the king where it is counts as a move. Cf. A094061. - Peter Bala, Feb 14 2017 LINKS G. C. Greubel, Table of n, a(n) for n = 0..1000 FORMULA a(n) = A002426(n)^2. G.f.: hypergeom([1/12, 5/12],[1],1728*x^4*(x-1)*(9*x-1)*(3*x+1)^2/(81*x^4-36*x^3-26*x^2-4*x+1)^3)/(81*x^4-36*x^3-26*x^2-4*x+1)^(1/4).  - Mark van Hoeij, May 07 2013 G.f.: 1 / AGM(1+3*x, sqrt((1-x)*(1-9*x))), where AGM(x,y) = AGM((x+y)/2,sqrt(x*y)) is the arithmetic-geometric mean. - Paul D. Hanna, Sep 04 2014 G.f.: 1 / AGM((1-x)*(1-3*x), (1+x)*(1+3*x)) = Sum_{n>=0} a(n)*x^(2*n). - Paul D. Hanna, Oct 04 2014 a(n) = (-1)^n*hypergeom([1/2,-n],[1],4)*hypergeom([(1-n)/2,-n/2],[1],4). - Peter Luschny, Nov 10 2014 a(n) ~ 3^(2*n+1) / (4*Pi*n). - Vaclav Kotesovec, Sep 28 2019 MAPLE a := n -> (-1)^n*hypergeom([1/2, -n], [1], 4)*hypergeom([1/2-n/2, -n/2], [1], 4): seq(simplify(a(n)), n=0..20); # Peter Luschny, Nov 10 2014 MATHEMATICA Table[(-1)^n*Hypergeometric2F1[1/2, -n, 1, 4] * Hypergeometric2F1[(1 - n)/2, -n/2, 1, 4], {n, 0, 50}] (* G. C. Greubel, Feb 26 2017 *) PROG (PARI) {a(n)=polcoeff((1+x+x^2 +x*O(x^n))^n, n)^2} for(n=0, 20, print1(a(n), ", ")) (PARI) /* Using AGM: */ {a(n)=polcoeff( 1 / agm(1+3*x, sqrt((1+3*x)^2 - 16*x +x*O(x^n))), n)} for(n=0, 20, print1(a(n), ", ")) \\ Paul D. Hanna, Sep 04 2014 CROSSREFS Cf. A002426, A133053, A168598, A243949, A094061. Sequence in context: A188235 A286810 A066558 * A169724 A135625 A160603 Adjacent sequences:  A168594 A168595 A168596 * A168598 A168599 A168600 KEYWORD nonn,easy AUTHOR Paul D. Hanna, Dec 01 2009 STATUS approved

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Last modified February 24 13:05 EST 2020. Contains 332209 sequences. (Running on oeis4.)