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 A070443 a(n) = n^2 mod 21. 2
 0, 1, 4, 9, 16, 4, 15, 7, 1, 18, 16, 16, 18, 1, 7, 15, 4, 16, 9, 4, 1, 0, 1, 4, 9, 16, 4, 15, 7, 1, 18, 16, 16, 18, 1, 7, 15, 4, 16, 9, 4, 1, 0, 1, 4, 9, 16, 4, 15, 7, 1, 18, 16, 16, 18, 1, 7, 15, 4, 16, 9, 4, 1, 0, 1, 4, 9, 16, 4, 15, 7, 1, 18, 16, 16, 18, 1, 7, 15, 4, 16, 9, 4, 1, 0, 1, 4, 9 (list; graph; refs; listen; history; text; internal format)
 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 Sequence in context: A070445 A070444 A120866 * A279403 A330377 A070643 Adjacent sequences:  A070440 A070441 A070442 * A070444 A070445 A070446 KEYWORD nonn,easy AUTHOR N. J. A. Sloane, May 12 2002 STATUS approved

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Last modified April 21 23:52 EDT 2021. Contains 343156 sequences. (Running on oeis4.)