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A317494
Triangle read by rows: T(0,0) = 1; T(n,k) = T(n-1,k) + 2 * T(n-3,k-1) for k = 0..floor(n/3); T(n,k)=0 for n or k < 0.
2
1, 1, 1, 1, 2, 1, 4, 1, 6, 1, 8, 4, 1, 10, 12, 1, 12, 24, 1, 14, 40, 8, 1, 16, 60, 32, 1, 18, 84, 80, 1, 20, 112, 160, 16, 1, 22, 144, 280, 80, 1, 24, 180, 448, 240, 1, 26, 220, 672, 560, 32, 1, 28, 264, 960, 1120, 192, 1, 30, 312, 1320, 2016, 672, 1, 32, 364, 1760, 3360, 1792, 64
OFFSET
0,5
COMMENTS
The numbers in rows of the triangle are along a "second layer" of skew diagonals pointing top-right in center-justified triangle given in A013609 ((1+2*x)^n) and along a "second layer" of skew diagonals pointing top-left in center-justified triangle given in A038207 ((2+x)^n), see links. (Note: First layer skew diagonals in center-justified triangles of coefficients in expansions of (1+2*x)^n and (2+x)^n are given in A128099 and A207538 respectively.)
The coefficients in the expansion of 1/(1-x-2x^3) are given by the sequence generated by the row sums.
The row sums give A003229.
If s(n) is the row sum at n, then the ratio s(n)/s(n-1) is approximately 1.695620769559862... (see A289265), when n approaches infinity.
REFERENCES
Shara Lalo and Zagros Lalo, Polynomial Expansion Theorems and Number Triangles, Zana Publishing, 2018, ISBN: 978-1-9995914-0-3, pp. 358, 359
FORMULA
T(n,k) = 2^k / ((n - 3k)! k!) * (n - 2k)! where n is a nonnegative integer and k = 0..floor(n/3).
EXAMPLE
Triangle begins:
1;
1;
1;
1, 2;
1, 4;
1, 6;
1, 8, 4;
1, 10, 12;
1, 12, 24;
1, 14, 40, 8;
1, 16, 60, 32;
1, 18, 84, 80;
1, 20, 112, 160, 16;
1, 22, 144, 280, 80;
1, 24, 180, 448, 240;
1, 26, 220, 672, 560, 32;
1, 28, 264, 960, 1120, 192;
1, 30, 312, 1320, 2016, 672;
1, 32, 364, 1760, 3360, 1792, 64;
MATHEMATICA
t[n_, k_] := t[n, k] = 2^k/((n - 3 k)! k!) (n - 2 k)!; Table[t[n, k], {n, 0, 18}, {k, 0, Floor[n/3]} ] // Flatten.
t[0, 0] = 1; t[n_, k_] := t[n, k] = If[n < 0 || k < 0, 0, t[n - 1, k] + 2 t[n - 3, k - 1]]; Table[t[n, k], {n, 0, 18}, {k, 0, Floor[n/3]}] // Flatten.
PROG
(GAP) Flat(List([0..20], n->List([0..Int(n/3)], k->2^k/(Factorial(n-3*k)*Factorial(k))*Factorial(n-2*k)))); # Muniru A Asiru, Jul 31 2018
CROSSREFS
Row sums give A003229.
Sequence in context: A318775 A317500 A339420 * A317505 A137374 A131516
KEYWORD
tabf,nonn,easy
AUTHOR
Zagros Lalo, Jul 30 2018
STATUS
approved