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 A004247 Multiplication table read by antidiagonals: T(i,j) = i*j (i>=0, j>=0). Alternatively, multiplication triangle read by rows: P(i,j) = j*(i-j) (i>=0, 0<=j<=i). 25
 0, 0, 0, 0, 1, 0, 0, 2, 2, 0, 0, 3, 4, 3, 0, 0, 4, 6, 6, 4, 0, 0, 5, 8, 9, 8, 5, 0, 0, 6, 10, 12, 12, 10, 6, 0, 0, 7, 12, 15, 16, 15, 12, 7, 0, 0, 8, 14, 18, 20, 20, 18, 14, 8, 0, 0, 9, 16, 21, 24, 25, 24, 21, 16, 9, 0, 0, 10, 18, 24, 28, 30, 30, 28, 24, 18, 10, 0, 0, 11, 20, 27, 32, 35, 36, 35, 32, 27, 20, 11, 0, 0, 12, 22, 30, 36, 40, 42, 42, 40, 36, 30 (list; table; graph; refs; listen; history; text; internal format)
 OFFSET 0,8 COMMENTS Table of x*y, where (x,y) = (0,0),(0,1),(1,0),(0,2),(1,1),(2,0),... Or, triangle read by rows, in which row n gives the numbers 0, n*1, (n-1)*2, (n-2)*3, ..., 2*(n-1), 1*n, 0. Letting T(n,k) be the (k+1)st entry in the (n+1)st row (same numbering used for Pascal's triangle), T(n,k) is the dimension of the space of all k-dimensional subspaces of a (fixed) n-dimensional real vector space. - Paul Boddington, Oct 21 2003 From Dennis P. Walsh, Nov 10 2009: (Start) Triangle P(n,k), 0<=k<=n, equals n^2 x the variance of a binary data set with k zeros and (n-k) ones. [For the case when n=0, let the variance of the empty set be defined as 0.] P(n,k) is also the number of ways to form an opposite-sex dance couple from k women and (n-k) men. (End) P(n,k) is the number of negative products of two numbers from a set of n real numbers, k of which are negative. - Logan Pipes, Jul 08 2021 LINKS T. D. Noe, Rows n = 0..50 of triangle, flattened Dennis Walsh, Variance bounds on binary data sets FORMULA a(n) = A002262(n) * A025581(n). - Antti Karttunen From Ridouane Oudra, Dec 14 2019: (Start) a(n) = A004197(n)*A003984(n). a(n) = (3/4 + n)*t^2 - (1/4)*t^4 - (1/2)*t - n^2 - n, where t = floor(sqrt(2*n+1)+1/2). (End) P(n,k) = (P(n-1,k-1) + P(n-1,k) + n) / 2. - Robert FERREOL, Jan 16 2020 P(n,floor(n/2)) = A002620(n). - Logan Pipes, Jul 08 2021 EXAMPLE As the triangle P, sequence begins:   0;   0,0;   0,1,0;   0,2,2,0;   0,3,4,3,0;   0,4,6,6,4,0,;   0,5,8,9,8,5,0;   ... From Dennis P. Walsh, Nov 10 2009: (Start) P(5,2)=T(2,3)=6 since the variance of the data set <0,0,1,1,1> equals 6/25. P(5,2)=6 since, with 2 women, say Alice and Betty, and with 3 men, say Charles, Dennis, and Ed, the dance couple is one of the following: {Alice, Charles}, {Alice, Dennis}, {Alice, Ed}, {Betty, Charles}, {Betty, Dennis} and {Betty, Ed}. (End) MAPLE seq(seq(k*(n-k), k=0..n), n=0..13); # Dennis P. Walsh, Nov 10 2009 MATHEMATICA Table[(x - y) y, {x, 0, 13}, {y, 0, x}] // Flatten (* Robert G. Wilson v, Oct 06 2007 *) PROG (PARI) T(i, j)=i*j \\ Charles R Greathouse IV, Jun 23 2017 CROSSREFS See A003991 for another version with many more comments. Cf. A002262, A025581, A003056, A004197, A003984, A048720, A325820, A000292 (row sums of triangle). Sequence in context: A067138 A059692 A336225 * A271916 A327031 A014473 Adjacent sequences:  A004244 A004245 A004246 * A004248 A004249 A004250 KEYWORD tabl,nonn,easy,nice AUTHOR EXTENSIONS Edited by N. J. A. Sloane, Sep 30 2007 STATUS approved

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Last modified December 7 13:08 EST 2021. Contains 349581 sequences. (Running on oeis4.)