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Positive integers n such that (n+57)^3 - n^3 is a square.
8

%I #9 Mar 05 2024 05:08:32

%S 551,13471,67002512,1560515752,7745359676111,180392503180711,

%T 895348087775371352,20853012581126608912,103500448242912021166871,

%U 2410566548172681237123151,11964444815088795735075876992,278656671814812593067838694872,1383065891631134161140389210648831

%N Positive integers n such that (n+57)^3 - n^3 is a square.

%H Colin Barker, <a href="/A263947/b263947.txt">Table of n, a(n) for n = 1..394</a>

%H <a href="/index/Rec#order_05">Index entries for linear recurrences with constant coefficients</a>, signature (1,115598,-115598,-1,1).

%F a(n) = a(n-1)+115598*a(n-2)-115598*a(n-3)-a(n-4)+a(n-5) for n>5.

%F G.f.: 19*x*(32*x^4+680*x^3-173397*x^2-680*x-29) / ((x-1)*(x^2-340*x+1)*(x^2+340*x+1)).

%e 551 is in the sequence because (551+57)^3 - 551^3 = 7581^2.

%t LinearRecurrence[{1, 115598, -115598, -1, 1}, {551, 13471, 67002512, 1560515752, 7745359676111}, 15] (* _Paolo Xausa_, Mar 05 2024 *)

%o (PARI) Vec(19*x*(32*x^4+680*x^3-173397*x^2-680*x-29)/((x-1)*(x^2-340*x+1)*(x^2+340*x+1)) + O(x^20))

%Y Cf. A263942 (4), A263943 (21), A263944 (28), A263945 (39), A263946 (52), A263948 (61), A263949 (84) where the parenthesized number is k in the expression (n+k)^3 - n^3.

%K nonn,easy

%O 1,1

%A _Colin Barker_, Oct 30 2015