OFFSET

1,5

COMMENTS

Column k, except for the initial k-1 0's, is an arithmetic progression with first term 1 and common difference 2(k-1). Row sums yield A116731. First column of the inverse matrix is A129779.

Studied by Paul Curtz circa 1993.

From Rogério Serôdio, Dec 19 2017: (Start)

T(n, k) gives the number of distinct sums of 2(k-1) elements in {1,1,2,2,...,n-1,n-1}. For example, T(6, 2) = the number of distinct sums of 2 elements in {1,1,2,2,3,3,4,4,5,5}, and because each sum from the smallest 1 + 1 = 2 to the largest 5 + 5 = 10 appears, T(6, 2) = 10 - 1 = 9. [In general:

2*Sum_{j=1..(k-1)} (n-j) - (2*(Sum_{j=1..k-1} j) - 1) = 2*(n*(k-1) - 4*(k-1)*k/2 + 1 = 2*(k-1)*(n-k) + 1 = T(n, k). - Wolfdieter Lang, Dec 20 2017]

T(n, k) is the number of lattice points with abscissa x = 2*(k-1) and even ordinate in the closed region bounded by the parabola y = x*(2*(n-1) - x) and the x axis. [That is, (1/2)*y(2*(k-1)) + 1 = T(n, k). - Wolfdieter Lang, Dec 20 2017]

LINKS

G. C. Greubel, Rows n = 1..100 of triangle, flattened

FORMULA

T(n, k) = 1 + 2*(n-k)*(k-1) (1 <= k <= n).

G.f.: G(t,z) = t*z*(3*t*z^2 - z - t*z + 1)/((1-t*z)*(1-z))^2.

Equals = 2 * A077028 - A000012 as infinite lower triangular matrices. - Gary W. Adamson, Oct 23 2007

T(n, 1) = 1 and T(n, n) = 1 for n >= 1; T(n, k) = (T(n-1, k-1)*T(n-1, k) + 2)/T(n-2, k-1), for n > 2 and 1 < k < n. See a comment above. - Rogério Serôdio, Dec 19 2017

G.f. column k (with leading zeros): (x^k/(1-x)^2)*(1 + (2*k-3)*x), k >= 1. See the g.f. of the triangle G(t,z) above: (d/dt)^k G(t,x)/k!|_{t=0}. - Wolfdieter Lang, Dec 20 2017

EXAMPLE

The triangle T(n, k) starts:

n\k 1 2 3 4 5 6 7 8 9 10 ...

1: 1

2: 1 1

3: 1 3 1

4: 1 5 5 1

5: 1 7 9 7 1

6: 1 9 13 13 9 1

7: 1 11 17 19 17 11 1

8: 1 13 21 25 25 21 13 1

9: 1 15 25 31 33 31 25 15 1

10: 1 17 29 37 41 41 37 29 17 1

... reformatted. - Wolfdieter Lang, Dec 19 2017

MAPLE

T:=proc(n, k) if k<=n then 2*(n-k)*(k-1)+1 else 0 fi end: for n from 1 to 14 do seq(T(n, k), k=1..n) od; # yields sequence in triangular form

MATHEMATICA

Flatten[Table[1+2(n-k)(k-1), {n, 0, 20}, {k, n}]] (* Harvey P. Dale, Jul 13 2013 *)

PROG

(PARI) T(n, k) = 1 + 2*(n-k)*(k-1) \\ Iain Fox, Dec 19 2017

(PARI) first(n) = my(res = vector(binomial(n+1, 2)), i = 1); for(r=1, n, for(k=1, r, res[i] = 1 + 2*(r-k)*(k-1); i++)); res \\ Iain Fox, Dec 19 2017

(Magma) [1 + 2*(n-k)*(k-1): k in [1..n], n in [1..12]]; // G. C. Greubel, Nov 25 2019

(Sage) [[1 + 2*(n-k)*(k-1) for k in (1..n)] for n in (1..12)] # G. C. Greubel, Nov 25 2019

(GAP) Flat(List([1..12], n-> List([1..n], k-> 1 + 2*(n-k)*(k-1) ))); # G. C. Greubel, Nov 25 2019

CROSSREFS

KEYWORD

AUTHOR

Emeric Deutsch, May 22 2007

EXTENSIONS

Edited by Wolfdieter Lang, Dec 19 2017

STATUS

approved