OFFSET
1,1
COMMENTS
REFERENCES
"Supplemento al Periodico di Matematica", Raffaello Giusti Editore (Livorno) - Mar 1901 - p. 75 (Problem 286 and its generalization, G. Cardoso-Laynes).
LINKS
Mohammed Yaseen, Table of n, a(n) for n = 1..10000 (first 1000 terms from Bruno Berselli)
Index entries for linear recurrences with constant coefficients, signature (1,2,-2,-1,1).
FORMULA
MAPLE
select(t -> issqr(t/3), [seq(20*i+7, i=1..10000, 3)]); # Robert Israel, Apr 28 2023
MATHEMATICA
Select[20 Range[5000] + 7, IntegerQ[Sqrt[#/3]] &] (* or *) LinearRecurrence[{1, 2, -2, -1, 1}, {27, 147, 507, 867, 1587}, 40] (* Harvey P. Dale, Jul 06 2011 *)
CoefficientList[Series[3 (9 + 40 x + 102 x^2 + 40 x^3 + 9 x^4) / ((1 + x)^2 (1 - x)^3), {x, 0, 35}], x] (* Vincenzo Librandi, Aug 19 2013 *)
PROG
(Magma) [m: m in [7..10^5 by 20] | IsSquare(m/3)];
(PARI) for(k=0, 5*10^3, m=20*k+7; if(issquare(m/3), print1(m", ")));
(PARI) a(n)=my(m=n--\4); 1200*m^2+[360*m+27, 840*m+147, 1560*m+507, 2040*m+867][n%4+1] \\ Charles R Greathouse IV, Jun 29 2011
CROSSREFS
KEYWORD
nonn,easy
AUTHOR
Bruno Berselli, Jun 28 2011
EXTENSIONS
Offset corrected by Mohammed Yaseen, Apr 27 2023
STATUS
approved