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 A026557 a(n) = T(n, n-4), T given by A026552. Also a(n) = number of integer strings s(0),...,s(n) counted by T, such that s(n)=4. 18
 1, 3, 12, 28, 93, 201, 631, 1316, 4037, 8259, 25052, 50680, 152782, 306958, 921982, 1844304, 5526849, 11024331, 32987492, 65675764, 196323853, 390374193, 1166171943, 2316881892, 6918228187, 13737041045, 41007165500 (list; graph; refs; listen; history; text; internal format)
 OFFSET 4,2 LINKS G. C. Greubel, Table of n, a(n) for n = 4..1000 FORMULA a(n) = A026552(n, n-4). MATHEMATICA T[n_, k_]:= T[n, k]= If[k==0 || k==2*n, 1, If[k==1 || k==2*n-1, Floor[(n+2)/2], If[EvenQ[n], T[n-1, k-2] + T[n-1, k] + T[n-1, k-1], T[n-1, k-2] + T[n-1, k]]]]; (* T=A026552 *) Table[T[n, n-4], {n, 4, 40}] (* G. C. Greubel, Dec 17 2021 *) PROG (Sage) @CachedFunction def T(n, k): # T = A026552 if (k==0 or k==2*n): return 1 elif (k==1 or k==2*n-1): return (n+2)//2 elif (n%2==0): return T(n-1, k) + T(n-1, k-1) + T(n-1, k-2) else: return T(n-1, k) + T(n-1, k-2) [T(n, n-4) for n in (4..40)] # G. C. Greubel, Dec 17 2021 CROSSREFS Cf. A026552, A026553, A026554, A026555, A026556, A026558, A026559, A026560, A026563, A026566, A026567, A027272, A027273, A027274, A027275, A027276. Sequence in context: A005995 A351643 A034503 * A124052 A215640 A104353 Adjacent sequences: A026554 A026555 A026556 * A026558 A026559 A026560 KEYWORD nonn AUTHOR Clark Kimberling STATUS approved

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Last modified December 10 22:05 EST 2023. Contains 367717 sequences. (Running on oeis4.)