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 A289741 a(n) = Kronecker symbol (-20/n). 7
 0, 1, 0, 1, 0, 0, 0, 1, 0, 1, 0, -1, 0, -1, 0, 0, 0, -1, 0, -1, 0, 1, 0, 1, 0, 0, 0, 1, 0, 1, 0, -1, 0, -1, 0, 0, 0, -1, 0, -1, 0, 1, 0, 1, 0, 0, 0, 1, 0, 1, 0, -1, 0, -1, 0, 0, 0, -1, 0, -1, 0, 1, 0, 1, 0, 0, 0, 1, 0, 1, 0, -1, 0, -1, 0, 0, 0, -1, 0, -1, 0 (list; graph; refs; listen; history; text; internal format)
 OFFSET 0,1 COMMENTS Period 20: repeat [0, 1, 0, 1, 0, 0, 0, 1, 0, 1, 0, -1, 0, -1, 0, 0, 0, -1, 0, -1]. This sequence is one of the three non-principal real Dirichlet characters modulo 20. The other two are Jacobi or Kronecker symbols {(20/n)} (or {(n/20)}) and {((-100)/n)} (A185276). Note that (Sum_{i=0..19} i*a(i))/(-20) = 2 gives the class number of the imaginary quadratic field Q(sqrt(-5)). The fact Q(sqrt(-5)) has class number 2 implies that Q(sqrt(-5)) is not a unique factorization domain. LINKS Antti Karttunen, Table of n, a(n) for n = 0..10000 Eric Weisstein's World of Mathematics, Kronecker Symbol Index entries for linear recurrences with constant coefficients, signature (0,0,0,0,0,0,0,0,0,-1). FORMULA a(n) = 1 for n in A045797; -1 for n in A045798; 0 for n that are not coprime with 20. Completely multiplicative with a(p) = a(p mod 20) for primes p. a(n) = A080891(n)*A101455(n). a(n) = -a(n+10) = -a(-n) for all n in Z. Multiplicative with a(2) = a(5) = 0, a(p) = (-1)^floor(p/10) otherwise; equivalently: a(n) = (-1)^floor(n/10) if n is coprime to 2*5, 0 otherwise. - M. F. Hasler, Feb 28 2022 MATHEMATICA Array[KroneckerSymbol[-20, #]&, 100, 0] (* Amiram Eldar, Jan 10 2019 *) PROG (PARI) a(n) = kronecker(-20, n) CROSSREFS Cf. A035170 (inverse Moebius transform). Kronecker symbols {(d/n)} where d is a fundamental discriminant with |d| <= 24: A109017 (d=-24), A011586 (d=-23), this sequence (d=-20), A011585 (d=-19), A316569 (d=-15), A011582 (d=-11), A188510 (d=-8), A175629 (d=-7), A101455 (d=-4), A102283 (d=-3), A080891 (d=5), A091337 (d=8), A110161 (d=12), A011583 (d=13), A011584 (d=17), A322829 (d=21), A322796 (d=24). Cf. A045797, A045798, A185276. Sequence in context: A353529 A286907 A284851 * A127266 A083923 A352824 Adjacent sequences: A289738 A289739 A289740 * A289742 A289743 A289744 KEYWORD sign,mult,easy AUTHOR Jianing Song, Dec 27 2018 STATUS approved

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