A026758 Triangular array T read by rows: T(n,0)=T(n,n)=1 for n >= 0; for n >= 2 and 1 <= k <= n-1, T(n,k) = T(n-1,k-1) + T(n-2,k-1) + T(n-1,k) if n is odd and 1 <= k <= (n-1)/2, else T(n,k) = T(n-1,k-1) + T(n-1,k).

1, 1, 1, 1, 2, 1, 1, 4, 3, 1, 1, 5, 7, 4, 1, 1, 7, 16, 11, 5, 1, 1, 8, 23, 27, 16, 6, 1, 1, 10, 38, 66, 43, 22, 7, 1, 1, 11, 48, 104, 109, 65, 29, 8, 1, 1, 13, 69, 190, 279, 174, 94, 37, 9, 1, 1, 14, 82, 259, 469, 453, 268, 131, 46, 10, 1, 1, 16, 109, 410, 918, 1201, 721, 399, 177, 56, 11, 1

Offset: 0

Author

Clark Kimberling

Keywords

• nonn
• tabl

Examples

			Triangle begins as:
  1;
  1,  1;
  1,  2,  1;
  1,  4,  3,  1;
  1,  5,  7,  4,  1;
  1,  7, 16, 11,  5,  1;
  1,  8, 23, 27, 16,  6, 1;
  1, 10, 38, 66, 43, 22, 7, 1;
		

Links

• G. C. Greubel, Rows n = 0..100 of triangle, flattened

Crossrefs

Cf. A026765 (row sums).

Programs

GAP

T:= function(n,k)
    if k=0 or k=n then return 1;
    elif (n mod 2)=1 and k List([0..n], k-> T(n,k) ))); # G. C. Greubel, Oct 29 2019

Maple

T:= proc(n,k) option remember;
   if k=0 or k = n then 1;
   elif type(n,'odd') and k <= (n-1)/2 then
        procname(n-1,k-1)+procname(n-2,k-1)+procname(n-1,k) ;
   else
       procname(n-1,k-1)+procname(n-1,k) ;
   end if ;
end proc;
seq(seq(T(n,k), k=0..n), n=0..12); # G. C. Greubel, Oct 29 2019

Mathematica

T[n_, k_]:= T[n, k]= If[k==0 || k==n, 1, If[OddQ[n] && k<=(n-1)/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[T[n, k], {n,0,12}, {k,0,n}]//Flatten (* G. C. Greubel, Oct 29 2019 *)

PARI

T(n,k) = if(k==0 || k==n, 1, if(n%2==1 && k<=(n-1)/2, T(n-1,k-1) + T(n-2,k-1) + T(n-1,k), T(n-1,k-1) + T(n-1,k) ));
for(n=0,12, for(k=0,n, print1(T(n,k), ", "))) \\ G. C. Greubel, Oct 29 2019

Sage

@CachedFunction
def T(n, k):
    if (k==0 or k==n): return 1
    elif (mod(n,2)==1 and k<=(n-1)/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)
[[T(n, k) for k in (0..n)] for n in (0..12)] # G. C. Greubel, Oct 29 2019

Formula

T(n, k) = number of paths from (0, 0) to (n-k, k) in directed graph having vertices (i, j) and edges (i, j)-to-(i+1, j) and (i, j)-to-(i, j+1) for i, j >= 0 and edges (i, 2h+i+1)-to-(i+1, 2h+i+2) for i >= 0, h>=0.

Extensions

Offset corrected by Sean A. Irvine, Oct 25 2019

More terms added by G. C. Greubel, Oct 29 2019