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 A086227 a(n)=Sum( i^k*tan(k*Pi/(4n)))/(4i) where 1<=k<=4n and (k,n)=1. 1
 -1, 2, -2, 2, -4, 4, -4, 6, -4, 6, -8, 6, -8, 8, -8, 8, -12, 10, -8, 16, -12, 12, -16, 10, -12, 18, -16, 14, -16, 16, -16, 24, -16, 16, -24, 18, -20, 24, -16, 20, -32, 22, -24, 24, -24, 24, -32, 28, -20, 32, -24, 26, -36, 24, -32, 40, -28, 30, -32, 30, -32, 48, -32, 24, -48, 34, -32, 48, -32, 36, -48, 36, -36, 40, -40, 48, -48 (list; graph; refs; listen; history; text; internal format)
 OFFSET 2,2 COMMENTS This seems to be (-1)^(n+1) times h(-4n^2) = (-1)^(n+1)*A000003(n^2), where h(k) is the class number. Verified for n <= 10^5. - Charles R Greathouse IV, Apr 28 2013 LINKS PROG (PARI) a(n)=round(real(1/4/I*sum(k=1, 4*n, (I^k)*tan(Pi/4/n*if(gcd(k, n)-1, 0, k))))) (PARI) a(n)=round(imag(sum(k=1, 4*n, if(gcd(k, n)==1, I^k*tan(k*Pi/4/n))))/4) \\ Charles R Greathouse IV, Apr 25 2013 (PARI) a(n)=my(s); for(k=1, 2*n, if(gcd(2*k-1, n)==1, s-=(-1)^k*tan((2*k-1)*Pi/4/n))); round(s/4) \\ Charles R Greathouse IV, Apr 25 2013 CROSSREFS Sequence in context: A054861 A187324 A323094 * A302402 A079438 A123050 Adjacent sequences:  A086224 A086225 A086226 * A086228 A086229 A086230 KEYWORD sign AUTHOR Benoit Cloitre, Aug 28 2003 EXTENSIONS Definition corrected by Charles R Greathouse IV, Apr 25 2013 STATUS approved

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Last modified June 18 18:52 EDT 2019. Contains 324215 sequences. (Running on oeis4.)