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 A002721 Number of 3 X 3 X 3 arrays M_ijk (1 <= i,j,k <= 3) with entries satisfying 0 <= M_ijk <= n and all line sums equal to n. (Formerly M4864 N2080) 40
 1, 12, 132, 847, 3921, 14506, 45402, 124707, 308407, 699766, 1477686, 2936517, 5540107, 9993192, 17333536, 29048541, 47220357, 74703832, 115341952, 174223731, 257989821, 375191422, 536708382, 756232687, 1050823851, 1441543026, 1954172962 (list; graph; refs; listen; history; text; internal format)
 OFFSET 0,2 COMMENTS Number of 3 X 3 X 3 arrays M_ijk (1 <= i,j,k <= 3) satisfying Sum_i M_ijk = n (all j,k), Sum_j M_ijk = n (all i,k), Sum_k M_ijk = n (all i,j) and 0 <= M_ijk <= n. The constraints imply that Sum_{i,j,k} M_ijk = 9n. This is a "magic cube" in Stanley's notation (see Stanley references). - N. J. A. Sloane, Jul 07 2014 REFERENCES N. J. A. Sloane, A Handbook of Integer Sequences, Academic Press, 1973 (includes this sequence). N. J. A. Sloane and Simon Plouffe, The Encyclopedia of Integer Sequences, Academic Press, 1995 (includes this sequence). R. P. Stanley, Enumerative Combinatorics, Wadsworth, Vol. 1, Second Edition, Section 4.6.1. LINKS Vincenzo Librandi, Table of n, a(n) for n = 0..1000 A. G. Bell, Partitioning integers in n dimensions, The Computer Journal, 13 (1970), 278-283. R. P. Stanley, Examples of Magic Labelings, Unpublished Notes, 1973 [Cached copy, with permission] Index entries for linear recurrences with constant coefficients, signature (9,-36,84,-126,126,-84,36,-9,1). FORMULA a(n) = (1/4032) * m * (m * (m * (31 * m + 1004) + 6820) + 4272) + 1, where m = n*(n+1) (from the Bell reference). - Sean A. Irvine, Jul 01 2014 G.f.: -(x^8+3*x^7+60*x^6+7*x^5+168*x^4+7*x^3+60*x^2+3*x+1) / (x-1)^9. - Colin Barker, Jul 01 2014 EXAMPLE Comment from N. J. A. Sloane, Jul 06 2014. Here are four of the twelve arrays showing that a(1) = 12 (each row shows top face, middle face, bottom face): ----------- 100 010 001 010 001 100 001 100 010 ----------- 100 001 010 010 100 001 001 010 100 ----------- 001 010 100 010 100 001 100 001 010 ----------- 001 100 010 010 001 100 100 010 001 ----------- Each face must show one of the six 3 X 3 permutation matrices. There are 6 choices for the top face, and for each of these there are two choices for the second face and the third face is then determined, for a total of a(1)=6*2*1=12. MAPLE A002721:=n->(1/4032)*n*(n+1)*(n*(n+1)*(n*(n+1)*(31*n*(n+1)+1004)+6820)+ 4272)+1: seq(A002721(n), n=0..30); # Wesley Ivan Hurt, Jul 01 2014 MATHEMATICA CoefficientList[Series[-(x^8 + 3*x^7 + 60*x^6 + 7*x^5 + 168*x^4 + 7*x^3 + 60*x^2 + 3*x + 1)/(x - 1)^9, {x, 0, 30}], x] (* Wesley Ivan Hurt, Jul 01 2014 *) PROG (PARI) Vec(-(x^8+3*x^7+60*x^6+7*x^5+168*x^4+7*x^3+60*x^2+3*x+1)/(x-1)^9 + O(x^100)) \\ Colin Barker, Jul 01 2014 (Magma) [(1/4032)*n*(n+1)*(n*(n+1)*(n*(n+1)*(31*n*(n+1)+1004)+6820)+4272)+1 : n in [0..30] ]; // Wesley Ivan Hurt, Jul 01 2014 CROSSREFS See A001496 for the two-dimensional 4 X 4 analog. Cf. also A002817. Sequence in context: A305261 A048643 A111085 * A340508 A119217 A358113 Adjacent sequences: A002718 A002719 A002720 * A002722 A002723 A002724 KEYWORD nonn,easy AUTHOR N. J. A. Sloane. EXTENSIONS More terms from Sean A. Irvine, Jul 01 2014 Edited by N. J. A. Sloane, Jul 06 2014 STATUS approved

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