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A211790 Rectangular array:  R(k,n) = number of ordered triples (w,x,y) with all terms in {1,...,n} and w^k<x^k+y<k. 18

%I

%S 1,7,1,23,7,1,54,22,7,1,105,51,22,7,1,181,97,50,22,7,1,287,166,96,50,

%T 22,7,1,428,263,163,95,50,22,7,1,609,391,255,161,95,50,22,7,1,835,554,

%U 378,253,161,95,50,22,7,1,1111,756,534,374,252,161,95,50,22,7

%N Rectangular array: R(k,n) = number of ordered triples (w,x,y) with all terms in {1,...,n} and w^k<x^k+y<k.

%C Row 1: A004068

%C Row 2: A211635

%C Row 3: A211650

%C Limiting row sequence: A002412

%C ...

%C Let R be the array in A211790 and let R' be the array in A211793. Then R(k,n)+R'(k,n)=3^(n-1). Moreover, (row k of R) =(row k of A211796) for k>2, by Fermat's last theorem; likewise, (row k of R')=(row k of A211799) for k>2.

%C ...

%C Generalizations: Suppose that b,c,d are nonzero integers, and let U(k,n) be the number of ordered triples (w,x,y) with all terms in {1,...,n} and b*w*k <R> c*x^k+d*y^k, where the relation <R> is one of these: <, >=, <=, >. What additional assumptions force the limiting row sequence to be essentially one of these: A002412, A000330, A016061, A174723, A051925?

%C In the following guide to related arrays and sequences, U(k,n) denotes the number of (w,x,y) as described in the preceding paragraph:

%C A211790: w^k < x^k+y^k

%C first 3 rows: A004068, A211635, A211650

%C limiting row sequence: A002412

%C A211793: w^k >= x^k+y^k

%C first 3 rows: A000292, A211636, A211651

%C limiting row sequence: A000330

%C A211796: w^k <= x^k+y^k

%C first 3 rows: A002413, A211634, A211650

%C limiting row sequence: A002412

%C A211799: w^k > x^k+y^k

%C first 3 rows: A000292, A211637, A211651

%C limiting row sequence: A000330

%C A211802: 2w^k < x^k+y^k

%C first 3 rows: A182260, A211800, A211801

%C limiting row sequence: A016061

%C A211805: 2w^k >= x^k+y^k

%C first 3 rows: A055232, A211803, A211804

%C limiting row sequence: A000330

%C A211808: 2w^k <= x^k+y^k

%C first 3 rows: A055232, A211806, A211807

%C limiting row sequence: A174723

%C A182259: 2w^k > x^k+y^k

%C first 3 rows: A182260, A211810, A211811

%C limiting row sequence: A051925

%F R(k,n)=n(n-1)(4n+1)/6 for 1<=k<=n, and

%F R(k,n)=Sum{Sum{floor[(x^k+y^k)^(1/k)] : 1<=x<=n, 1<=y<=n}} for 1<=k<=n.

%e Northwest corner:

%e 1...7...23...54...105...181...287...428...609

%e 1...7...22...51...97....166...263...391...554

%e 1...7...22...50...96....163...255...378...534

%e 1...7...22...50...95....161...253...374...528

%e 1...7...22...50...95....161...252...373...527

%e For n=2 and k>=1, the 7 triples (w,x,y) are (1,1,1), (1,1,2), (1,2,1), (1,2,2), (2,1,2), (2,2,1), (2,2,2).

%t z = 48;

%t t[k_, n_] := Module[{s = 0},

%t (Do[If[w^k < x^k + y^k, s = s + 1],

%t {w, 1, #}, {x, 1, #}, {y, 1, #}] &[n]; s)];

%t Table[t[1, n], {n, 1, z}] (* A004068 *)

%t Table[t[2, n], {n, 1, z}] (* A211635 *)

%t Table[t[3, n], {n, 1, z}] (* A211650 *)

%t TableForm[Table[t[k, n], {k, 1, 12}, {n, 1, 16}]]

%t Flatten[Table[t[k, n - k + 1], {n, 1, 12}, {k, 1, n}]] (* A211790 *)

%t Table[n (n + 1) (4 n - 1)/6,

%t {n, 1, z}] (* row-limit sequence, A002412 *)

%t (* _Peter J. C. Moses_, Apr 13 2012 *)

%Y Cf. A211793, A211796, A211799, A211802, A211805, A211808, A182259

%K nonn,tabl

%O 1,2

%A _Clark Kimberling_, Apr 21 2012

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Last modified April 15 21:48 EDT 2021. Contains 343007 sequences. (Running on oeis4.)