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 A061857 Triangle in which the k-th item in the n-th row (both starting from 1) is the number of ways in which we can add 2 distinct integers from 1 to n in such a way that the sum is divisible by k. 8
 0, 1, 0, 3, 1, 1, 6, 2, 2, 1, 10, 4, 4, 2, 2, 15, 6, 5, 3, 3, 2, 21, 9, 7, 5, 4, 3, 3, 28, 12, 10, 6, 6, 4, 4, 3, 36, 16, 12, 8, 8, 5, 5, 4, 4, 45, 20, 15, 10, 9, 7, 6, 5, 5, 4, 55, 25, 19, 13, 11, 9, 8, 6, 6, 5, 5, 66, 30, 22, 15, 13, 10, 10, 7, 7, 6, 6, 5, 78, 36, 26, 18, 16, 12, 12, 9, 8, 7, 7 (list; table; graph; refs; listen; history; text; internal format)
 OFFSET 1,4 COMMENTS Since the sum of two distinct integers from 1 to n can be as much as 2n-1, this triangular table cannot show all the possible cases. For larger triangles showing all solutions, see A220691 and A220693. - Antti Karttunen, Feb 18 2013 [based on Robert Israel's mail, May 07 2012] LINKS Reinhard Zumkeller, Rows n = 1..150 of triangle, flattened Stackexchange, Question 142323 FORMULA From Robert Israel, May 08 2012: (Start) Let n+1 = b mod k with 0 <= b < k, q = (n+1-b)/k.  Let k = c mod 2, c = 0 or 1. If b = 0 or 1 then a(n,k) = q^2*k/2 + q*b - 2*q - b + 1 + c*q/2. If b >= (k+3)/2 then a(n,k) = q^2*k/2 + q*b - 2*q + b - 1 - k/2 + c*(q+1)/2. Otherwise a(n,k) = q^2*k/2 + q*b - 2*q + c*q/2. (End) EXAMPLE The second term on the sixth row is 6 because we have 6 solutions: {1+3, 1+5, 2+4, 2+6, 3+5, 4+6} and the third term on the same row is 5 because we have solutions {1+2,1+5,2+4,3+6,4+5}. Triangle begins:    0;    1,  0;    3,  1,  1;    6,  2,  2,  1;   10,  4,  4,  2,  2;   15,  6,  5,  3,  3,  2;   21,  9,  7,  5,  4,  3,  3;   28, 12, 10,  6,  6,  4,  4,  3;   36, 16, 12,  8,  8,  5,  5,  4,  4;   45, 20, 15, 10,  9,  7,  6,  5,  5,  4; MAPLE [seq(DivSumChoose2Triangle(j), j=1..120)]; DivSumChoose2Triangle := (n) -> nops(DivSumChoose2(trinv(n-1), (n-((trinv(n-1)*(trinv(n-1)-1))/2)))); DivSumChoose2 := proc(n, k) local a, i, j; a := []; for i from 1 to (n-1) do for j from (i+1) to n do if(0 = ((i+j) mod k)) then a := [op(a), [i, j]]; fi; od; od; RETURN(a); end; MATHEMATICA a[n_, 1] := n*(n-1)/2; a[n_, k_] := Module[{r}, r = Reduce[1 <= i < j <= n && Mod[i + j, k] == 0, {i, j}, Integers]; Which[Head[r] === Or, Length[r], Head[r] === And, 1, r === False, 0, True, Print[r, " not parsed"]]]; Table[a[n, k], {n, 1, 13}, {k, 1, n}] // Flatten (* Jean-François Alcover, Mar 04 2014 *) PROG (Haskell) a061857 n k = length [()| i <- [2..n], j <- [1..i-1], mod (i + j) k == 0] a061857_row n = map (a061857 n) [1..n] a061857_tabl = map a061857_row [1..] -- Reinhard Zumkeller, May 08 2012 (Scheme): (define (A061857 n) (A220691bi (A002024 n) (A002260 n))) - Antti Karttunen, Feb 18 2013. Needs A220691bi from A220691. CROSSREFS This is the lower triangular region of square array A220691. See A220693 for all nonzero solutions. The left edge (first diagonal) of the triangle: A000217, the second diagonal is given by C(((n+(n mod 2))/2), 2)+C(((n-(n mod 2))/2), 2) = A002620, the third diagonal by A058212, the fourth by A001971, the central column by A042963? trinv is given at A054425. Cf. A061865. Sequence in context: A162315 A109446 A088441 * A275464 A067433 A256697 Adjacent sequences:  A061854 A061855 A061856 * A061858 A061859 A061860 KEYWORD nonn,tabl AUTHOR Antti Karttunen, May 11 2001 EXTENSIONS Offset corrected by Reinhard Zumkeller, May 08 2012 STATUS approved

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Last modified September 27 10:36 EDT 2022. Contains 357057 sequences. (Running on oeis4.)