A135876 - OEIS

A135876

Square array, read by antidiagonals, where row n+1 is generated from row n by first removing terms at positions [(m+2)^2/4 - 1] for m>=0 and then taking partial sums, starting with all 1's in row 0.

1, 1, 1, 3, 2, 1, 15, 8, 3, 1, 105, 48, 15, 4, 1, 945, 384, 105, 23, 5, 1, 10395, 3840, 945, 176, 33, 6, 1, 135135, 46080, 10395, 1689, 279, 44, 7, 1, 2027025, 645120, 135135, 19524, 2895, 400, 57, 8, 1, 34459425, 10321920, 2027025, 264207, 35685, 4384, 561

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OFFSET

0,4

COMMENTS

This is the double factorial analog of Moessner's factorial array (A125714). Compare to triangle A135877 which is generated by a complementary process. A very interesting variant is square array A135878.

LINKS

Table of n, a(n) for n=0..51.

FORMULA

T(n,0) = (2n)!/n!/2^n; T(n,1) = 2^n*n!; T(n,2) = (2n+1)!/n!/2^n; T(n,3) = A004041(n) = (2n+1)!/n!/2^n * Sum_{k=0..n} 1/(2k+1). T(n,4) = A129890(n) = 2^(n+1)*(n+1)! - (2n+1)!/n!/2^n = T(n+1,1)-T(n+1,0).

EXAMPLE

Square array begins:

(1),(1),1,(1),1,(1),1,1,(1),1,1,(1),1,1,1,(1),1,1,1,(1),1,1,1,1,(1),...;

(1),(2),3,(4),5,(6),7,8,(9),10,11,(12),13,14,15,(16),17,18,19,(20),...;

(3),(8),15,(23),33,(44),57,71,(86),103,121,(140),161,183,206,(230),..;

(15),(48),105,(176),279,(400),561,744,(950),1206,1489,(1800),2171,..;

(105),(384),945,(1689),2895,(4384),6555,9129,(12139),16161,20763,..;

(945),(3840),10395,(19524),35685,(56448),89055,129072,(177331),245778,...;

(10395),(46080),135135,(264207),509985,(836352),1381905,2071215,(2924172),.;

(135135),(645120),2027025,(4098240),8294895,(14026752),24137505,...; ...

where terms in parenthesis are removed before taking partial sums.

For example, to generate row 2 from row 1, remove terms at positions

{[(m+2)^2/4-1], m>=0} = [0,1,3,5,8,11,15,19,24,29,35,...] to obtain:

[3, 5, 7,8, 10,11, 13,14,15, 17,18,19, 21,22,23,24, 25,26,27,28, ...]

then take partial sums to get row 2:

[3, 8, 15,23, 33,44, 57,71,86, 103,121,140, 161,183,206,230, ...].

Repeating this process will generate all the rows of the triangle,

where column 0 will be the odd double factorials (A001147)

and column 1 will be the even double factorials (A000165).

PROG

(PARI) {T(n, k)=local(A=0, b=0, c=0, d=0); if(n==0, A=1, until(d>k, if(c==floor((b+2)^2/4)-1, b+=1, A+=T(n-1, c); d+=1); c+=1)); A}

CROSSREFS

Cf. columns: A001147, A000165, A004041, A129890; variants: A135878, A125714.

Sequence in context: A140709 A109282 A135902 * A136217 A166884 A136220

Adjacent sequences: A135873 A135874 A135875 * A135877 A135878 A135879

KEYWORD

nonn,tabl

AUTHOR

Paul D. Hanna, Dec 14 2007

STATUS

approved