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A355320
Irregular triangle T(n, k), n >= 0, -2*n <= k <= 2*n, read by rows; T(0, 0) = 1; for n > 0, T(n, k) is the sum of all terms in previous rows at one knight's move away.
4
1, 1, 0, 0, 0, 1, 1, 0, 0, 1, 2, 1, 0, 0, 1, 1, 0, 0, 2, 3, 2, 0, 2, 3, 2, 0, 0, 1, 1, 0, 0, 3, 4, 3, 1, 6, 8, 6, 1, 3, 4, 3, 0, 0, 1, 1, 0, 0, 4, 5, 4, 3, 12, 16, 12, 6, 12, 16, 12, 3, 4, 5, 4, 0, 0, 1, 1, 0, 0, 5, 6, 5, 6, 20, 27, 21, 18, 33, 44, 33, 18, 21, 27, 20, 6, 5, 6, 5, 0, 0, 1
OFFSET
0,11
COMMENTS
See A096608 for the right half of the triangle.
Odd terms form fractal patterns (see illustrations in Links section).
LINKS
Paolo Xausa, Table of n, a(n) for n = 0..10010 (rows 0..70 of triangle, flattened)
Rémy Sigrist, Representation of the odd terms for n = 0..2^11 (only the right half of the triangle is represented)
FORMULA
T(n, k) = A096608(n, abs(k)).
T(n, 0) = A096609(n).
T(n, 1) = A096610(n).
T(n, 2) = A096611(n).
T(n, n) = A096612(n).
T(n, 2*n) = 1.
T(n, 2*n-1) = T(n, 2*n-2) = 0 for any n > 0.
T(n, k) = T'(n-1, k-2) + T'(n-1, k+2) + T'(n-2, k-1) + T'(n-2, k+1) for n > 0 (where T' extends T with 0's outside its domain of definition).
T(n, -k) = T(n, k).
Sum_{k = -2*n..2*n} T(n, k) = A002605(n+1).
EXAMPLE
Triangle T(n, k) begins:
1
1 0 0 0 1
1 0 0 1 2 1 0 0 1
1 0 0 2 3 2 0 2 3 2 0 0 1
1 0 0 3 4 3 1 6 8 6 1 3 4 3 0 0 1
1 0 0 4 5 4 3 12 16 12 6 12 16 12 3 4 5 4 0 0 1
MATHEMATICA
A355320[rowmax_]:=Module[{T}, T[0, 0]=1; T[n_, k_]:=T[n, k]=If[k<=2n, T[n-1, Abs[k-2]]+T[n-2, Abs[k-1]]+T[n-1, k+2]+T[n-2, k+1], 0]; Table[T[n, Abs[k]], {n, 0, rowmax}, {k, -2n, 2n}]]; A355320[10] (* Generates 11 rows *) (* Paolo Xausa, May 09 2023 *)
PROG
(PARI) row(n) = { my (rr=0, r=1); for (k=1, n, [rr, r]=[r, r*(1+'X^4)+rr*('X^3+'X^5)]); Vec(r) }
CROSSREFS
KEYWORD
nonn,tabf,nice,look
AUTHOR
Rémy Sigrist, Jun 28 2022
STATUS
approved