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 A027216 a(n) = Sum_{k=0..n-1} T(n,k)*T(n,k+1), T given by A026736. 1
 1, 4, 15, 63, 237, 1034, 3945, 17577, 67640, 304902, 1179415, 5352038, 20771331, 94628132, 368083879, 1680820301, 6548692260, 29946087674, 116816782997, 534628747310 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,2 LINKS G. C. Greubel, Table of n, a(n) for n = 1..1000 FORMULA a(n) ~ (1/2 - (-1)^n/10) * phi^(3*n - 5/2 + (-1)^n/2), where phi = A001622 = (1+sqrt(5))/2 is the golden ratio. - Vaclav Kotesovec, Jul 19 2019 MATHEMATICA T[n_, k_]:= T[n, k] = If[k==0 || k==n, 1, If[EvenQ[n] && k==(n-2)/2, T[n-1, k-1] + T[n-2, k-1] + T[n-1, k], T[n-1, k-1] + T[n-1, k]]]; Table[Sum[T[n, k]*T[n, k+1], {k, 0, n-1}], {n, 1, 30}] (* G. C. Greubel, Jul 19 2019 *) PROG (PARI) T(n, k) = if(k==n || k==0, 1, k==n-1, n, if((n%2)==0 && k==(n-2)/2, T(n-1, k-1) + T(n-2, k-1) + T(n-1, k), T(n-1, k-1) + T(n-1, k) )); vector(20, n, sum(k=0, n-1, T(n, k)*T(n, k+1)) ) \\ G. C. Greubel, Jul 19 2019 (Sage) @CachedFunction def T(n, k):     if (k==0 or k==n): return 1     elif (mod(n, 2)==0 and k==(n-2)/2): return T(n-1, k-1) + T(n-2, k-1) + T(n-1, k)     else: return T(n-1, k-1) + T(n-1, k) [sum(T(n, k)*T(n, k+1) for k in (0..n-1)) for n in (1..30)] # G. C. Greubel, Jul 19 2019 (GAP) T:= function(n, k)     if k=0 or k=n then return 1;     elif k=n-1 then return n;     elif (n mod 2)=0 and k=Int((n-2)/2) then return T(n-1, k-1) + T(n-2, k-1) + T(n-1, k);     else return T(n-1, k-1) + T(n-1, k);     fi;   end; List([1..20], n-> Sum([0..n-1], k-> T(n, k)*T(n, k+1) )); # G. C. Greubel, Jul 19 2019 CROSSREFS Cf. A026736. Sequence in context: A007161 A007167 A036728 * A124541 A323789 A007526 Adjacent sequences:  A027213 A027214 A027215 * A027217 A027218 A027219 KEYWORD nonn AUTHOR STATUS approved

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Last modified January 29 14:07 EST 2020. Contains 331338 sequences. (Running on oeis4.)