OFFSET
0,3
LINKS
G. C. Greubel, Table of n, a(n) for n = 0..1000
Sathwik Karnik, On the classification and algorithmic analysis of Carmichael numbers, arXiv:1702.08066 [math.NT], 2016. See Table 1.
Index entries for linear recurrences with constant coefficients, signature (0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 1).
FORMULA
From R. J. Mathar, Jul 27 2015: (Start)
a(n) = a(n-21).
G.f.: -x *(1+x) *(x^18 +3*x^17 +6*x^16 +10*x^15 -6*x^14 +21*x^13 -14*x^12 +15*x^11 +3*x^10 +13*x^9 +3*x^8 +15*x^7 -14*x^6 +21*x^5 -6*x^4 +10*x^3 +6*x^2 +3*x+1) ) / ( (x-1) *(1+x^6+x^5+x^4+x^3+x^2+x) *(1+x+x^2) *(1-x+x^3-x^4+x^6-x^8+x^9-x^11+x^12) ). (End)
MATHEMATICA
Table[Mod[n^2, 21], {n, 0, 200}] (* Vladimir Joseph Stephan Orlovsky, Apr 23 2011 *)
PowerMod[Range[0, 90], 2, 21] (* Harvey P. Dale, Jan 19 2013 *)
PROG
(PARI) a(n)=n^2%21 \\ Charles R Greathouse IV, Apr 06 2016
CROSSREFS
KEYWORD
nonn,easy
AUTHOR
N. J. A. Sloane, May 12 2002
STATUS
approved