OFFSET
1,1
COMMENTS
LINKS
Vincenzo Librandi, Table of n, a(n) for n = 1..1000
Index entries for linear recurrences with constant coefficients, signature (3,-3,1).
FORMULA
G.f.: -(28-57*x+27*x^2+8*x^6-11*x^7+3*x^9)/(1-x)^3.
EXAMPLE
For n = -19, n^3+28*n^2 = -6859+10108 = 3249 = 57^2 is a square.
For n = 0, n^3+28*n^2 = 0^3+28*0^2 = 0 = 0^2 is a square.
For n = 21; n^3+28*n^2 = 9261+12348 = 21609 = 147^2 is a square.
MATHEMATICA
CoefficientList[Series[-(28-57*x+27*x^2+8*x^6-11*x^7+3*x^9)/(1-x)^3, {x, 0, 60}], x] (* Vincenzo Librandi, Feb 22 2012 *)
Select[Range[-30, 2500], IntegerQ[Sqrt[#^3+28#^2]]&] (* or *) LinearRecurrence[ {3, -3, 1}, {-28, -27, -24, -19, -12, -3, 0, 8, 21, 36}, 60] (* Harvey P. Dale, Jan 10 2023 *)
PROG
(Magma) [ n: n in [ -30..2400] | IsSquare(n^3+28*n^2) ];
CROSSREFS
KEYWORD
sign
AUTHOR
Klaus Brockhaus, Jan 21 2009
STATUS
approved