OFFSET
1,1
COMMENTS
Numbers k such that k == 9 or 32 (mod 41). - Charles R Greathouse IV, Dec 27 2011
LINKS
Vincenzo Librandi, Table of n, a(n) for n = 1..1000
Index entries for linear recurrences with constant coefficients, signature (1,1,-1).
FORMULA
From M. F. Hasler, Jun 16 2010: (Start)
a(n) = 9*(-1)^(n+1) + 41*floor(n/2).
a(n) = a(n-2) + 41 for all n > 2. (End)
Sum_{n>=1} (-1)^(n+1)/a(n) = cot(9*Pi/41)*Pi/41. - Amiram Eldar, Feb 26 2023
MATHEMATICA
LinearRecurrence[{1, 1, -1}, {9, 32, 50}, 100] (* Vincenzo Librandi, Feb 29 2012 *)
Select[Range[1100], PowerMod[#, 2, 41] == 40 &] (* Vincenzo Librandi, Apr 24 2014 *)
PROG
(PARI) A155098(n)=n\2*41-9*(-1)^n /* M. F. Hasler, Jun 16 2010 */
CROSSREFS
KEYWORD
nonn,easy
AUTHOR
Vincenzo Librandi, Jan 20 2009
EXTENSIONS
Terms checked & minor edits by M. F. Hasler, Jun 16 2010
STATUS
approved