OFFSET
1,2
COMMENTS
Here m=(11*(2*n-1)+7*(-1)^n)/4 for n>0.
LINKS
B. Berselli, Table of n, a(n) for n = 1..10000.
Index entries for linear recurrences with constant coefficients, signature (1,2,-2,-1,1).
FORMULA
a(n) = (110*n*(n-1)+35*(2*n-1)*(-1)^n+39)/4.
G.f.: x*(1+90*x+38*x^2+90*x^3+1*x^4)/((1+x)^2*(1-x)^3).
a(n) = a(-n+1) = a(n-1)+2*a(n-2)-2*a(n-3)-a(n-4)+a(n-5).
MATHEMATICA
Select[(10Range[300]^2+1)/11, IntegerQ] (* or *) LinearRecurrence[ {1, 2, -2, -1, 1}, {1, 91, 131, 401, 481}, 50] (* Harvey P. Dale, Jul 10 2014 *)
PROG
(PARI) isok(n) = (((11*n-1) % 10 == 0) && issquare((11*n-1)/10)) \\ Michel Marcus, Jun 07 2013
(Magma) [(110*n*(n-1)+35*(2*n-1)*(-1)^n+39)/4: n in [1..50]]; // Vincenzo Librandi, Jul 10 2014
CROSSREFS
KEYWORD
nonn,easy
AUTHOR
Bruno Berselli, Jul 11 2010
STATUS
approved