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 A003991 Multiplication table read by antidiagonals: T(i,j) = i*j, i>=1, j>=1. 108
 1, 2, 2, 3, 4, 3, 4, 6, 6, 4, 5, 8, 9, 8, 5, 6, 10, 12, 12, 10, 6, 7, 12, 15, 16, 15, 12, 7, 8, 14, 18, 20, 20, 18, 14, 8, 9, 16, 21, 24, 25, 24, 21, 16, 9, 10, 18, 24, 28, 30, 30, 28, 24, 18, 10, 11, 20, 27, 32, 35, 36, 35, 32, 27, 20, 11, 12, 22, 30, 36, 40, 42, 42, 40, 36, 30, 22, 12 (list; table; graph; refs; listen; history; text; internal format)
 OFFSET 1,2 COMMENTS Or, triangle X(n,m) = T(n-m+1,m) read by rows, in which row n gives the numbers n*1, (n-1)*2, (n-2)*3, ..., 2*(n-1), 1*n. Radius of incircle of Pythagorean triangle with sides a=(n+1)^2-m^2, b=2*(n+1)*m and c=(n+1)^2+m^2. - Floor van Lamoen, Aug 16 2001 A permutation of A061017. - Matthew Vandermast, Feb 28 2003 In the proof of countability of rational numbers they are arranged in a square array. a(n) = p*q where p/q is the corresponding rational number as read from the array. - Amarnath Murthy, May 29 2003 Permanent of upper right n X n corner is A000442. - Marc LeBrun, Dec 11 2003 Row 12 gives total number of partridges, turtle doves, ... and drummers drumming that you have received at the end of the Twelve Days of Christmas song. - Alonso del Arte, Jun 17 2005 Consider a particle with spin S (a half-integer) and 2S+1 quantum states |m>, m = -S,-S+1,...,S-1,S. Then the matrix element = sqrt((S+m+1)(S-m)) of the spin-raising operator is the square-root of the triangular (tabl) element T(r,o) of this sequence in row r = 2S, and at offset o=2(S+m). T(r,o) is also the intensity || of the transition between the states |m> and |m+1>. For example, the five transitions between the 6 states of a spin S=5/2 particle have relative intensities 5,8,9,8,5. The total intensity of all spin 5/2 transitions (relative to spin 1/2) is 35, which is the tetrahedral number A000292(5). - Stanislav Sykora, May 26 2012 Sum_{k=0..2n-2} (-1)^k*a(A000124(2n-2)+k) = n. See A098359. - Charlie Marion, Apr 22 2013 T(n, k) is also the (k-1)-superdiagonal sum of an n X n Toeplitz matrix M(n) whose first row consists of successive positive integer numbers 1, ..., n. - Stefano Spezia, Jul 12 2019 REFERENCES J. H. Conway and R. K. Guy, The Book of Numbers, Copernicus Press, NY, 1996, p. 46. LINKS T. D. Noe, Rows n = 1..100 of triangle, flattened G. W. Leibniz, Dissertatio de arte combinatoria, 1666, Leipzig. (in Latin. This triangle appears on p. 208, page 44 of the PDF file). A. Necer, Séries formelles et produit de Hadamard, Journal de théorie des nombres de Bordeaux, 9 no. 2 (1997), p. 319-335. Boris Putievskiy, Transformations Integer Sequences And Pairing Functions arXiv:1212.2732 [math.CO], 2012. N. J. A. Sloane, "A Handbook of Integer Sequences" Fifty Years Later, arXiv:2301.03149 [math.NT], 2023, p. 5. FORMULA Rectangular array: T(n, m) = n*m, n>=1, m>= 1. Triangle X(n, m) = T(n-m+1, m) = (n-m+1)*m. Sum_{i=1..n} Sum_{j=1..n} a(n) = A000537(n) [Sum of first n cubes; or n-th triangular number squared.] Determinant of all n X n contiguous subarrays of A003991 is 0. - Gerald McGarvey, Sep 26 2004 G.f. as rectangular array: x * y / [ (1-x)^2 * (1-y)^2 ]. a(n) = i*j, where i=floor((1+sqrt(8n-7))/2), j=n-i*(i-1)/2. - Hieronymus Fischer, Aug 08 2007 As an infinite lower triangular matrix equals A000012 * A002260; where A000012 = (1; 1,1; 1,1,1; ...) and A002260 = (1; 1,2; 1,2,3; ...). - Gary W. Adamson, Oct 23 2007 As a linear array, the sequence is a(n) = A002260(n)*A004736(n) or a(n) = ((t*t+3*t+4)/2-n)*(n-(t*(t+1)/2)), where t=floor((-1+sqrt(8*n-7))/2). - Boris Putievskiy, Dec 17 2012 G.f. as linear array: (x - 3*x^2 + Sum_{k >= 0} ((k+2-x-(k+1)*x^2)*x^((k^2+3*k+4)/2)))/(1-x)^3. - Robert Israel, Dec 14 2015 E.g.f. as triangle: exp(x+y)*(1 + x - y + x*y - y^2). - Stefano Spezia, Jul 12 2019 a(n) = (1/2)*t + (n - 1/4)*t^2 - (1/4)*t^4 - n^2 + n, where t = floor(sqrt(2*n) + 1/2). - Ridouane Oudra, Nov 21 2020 a(n) = A003989(n) * A003990(n) = A059895(n) * A059896(n) = A059895(n)^2 * A059897(n). - Antti Karttunen, Dec 13 2021 T(n,k) = A002620(n+k) - A002620(n-k). - Michel Marcus, Jan 06 2023 EXAMPLE The array T starts in row n=1 with columns m>=1 as: 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 3 6 9 12 15 18 21 24 27 30 33 36 39 42 45 4 8 12 16 20 24 28 32 36 40 44 48 52 56 60 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 6 12 18 24 30 36 42 48 54 60 66 72 78 84 90 7 14 21 28 35 42 49 56 63 70 77 84 91 98 105 8 16 24 32 40 48 56 64 72 80 88 96 104 112 120 9 18 27 36 45 54 63 72 81 90 99 108 117 126 135 10 20 30 40 50 60 70 80 90 100 110 120 130 140 150 The triangle X(n, m) begins n\m 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 ... 1: 1 2: 2 2 3: 3 4 3 4: 4 6 6 4 5: 5 8 9 8 5 6: 6 10 12 12 10 6 7: 7 12 15 16 15 12 7 8: 8 14 18 20 20 18 14 8 9: 9 16 21 24 25 24 21 16 9 10: 10 18 24 28 30 30 28 24 18 10 11: 11 20 27 32 35 36 35 32 27 20 11 12: 12 22 30 36 40 42 42 40 36 30 22 12 13: 13 24 33 40 45 48 49 48 45 40 33 24 13 14: 14 26 36 44 50 54 56 56 54 50 44 36 26 14 15: 15 28 39 48 55 60 63 64 63 60 55 48 39 28 15 ... Formatted by Wolfdieter Lang, Dec 02 2014 MAPLE seq(seq(i*(n-i), i=1..n-1), n=2..10); # Robert Israel, Dec 14 2015 MATHEMATICA Table[(x + 1 - y) y, {x, 13}, {y, x}] // Flatten (* Robert G. Wilson v, Oct 06 2007 *) f[n_] := Table[SeriesCoefficient[E^(x + y) (1+ x - y +x*y-y^2), {x, 0, i}, {y, 0, j}]*i!*j!, {i, n, n}, {j, 0, n}]; Flatten[Array[f, 11, 0]] (* Stefano Spezia, Jul 12 2019 *) PROG (PARI) A003991(n, k) = if(k<1 || n<1, 0, k*n) (Magma) /* As triangle */ [[k*(n-k+1): k in [1..n]]: n in [1..15]]; // Vincenzo Librandi, Jul 12 2019 CROSSREFS Main diagonal gives squares A000290. Antidiagonal sums are tetrahedral numbers A000292. See A004247 for another version. Cf. A003989, A003990, A003056, A049581, A000442, A027424, A002260, A033638, A059895, A059896, A059897, A002620. Cf. also A051776, A067138, A091257, A325821, A331590, A341520, A350066. Sequence in context: A091257 A216622 A319840 * A131923 A119457 A241356 Adjacent sequences: A003988 A003989 A003990 * A003992 A003993 A003994 KEYWORD tabl,nonn,nice,easy,look AUTHOR Marc LeBrun EXTENSIONS More terms from Michael Somos STATUS approved

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Last modified May 27 20:43 EDT 2023. Contains 362987 sequences. (Running on oeis4.)