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 A319150 a(n) = gcd(A275286(n), A001818(n+1)) / A025549(n+1)^2. 0
 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 11, 11, 11, 11, 11, 11, 11, 11, 11, 11, 11, 1, 1, 1, 1, 1, 1, 1, 1, 1, 19, 19, 19, 19, 19, 19, 19, 19, 19, 19, 19, 19, 19, 19, 19, 19, 19, 19, 19, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1 (list; graph; refs; listen; history; text; internal format)
 OFFSET 0,28 COMMENTS A(n) = a(n)*A025549(n+1)^2 = gcd(B(n), C(n)). B(n) = A275286(n). C(n) = A001818(n+1). At first gcd(B(n), C(n)) = A025549(n+1)^2, but from n = 27 to n = 37, gcd(B(n), C(n)) = 11*A025549(n+1)^2, and then comes back to normal, then equals 19*A025549(n+1)^2, comes back to normal again, and so on ... Let S(n) = Sum_{k=0..n} ((-1)^k)/(2*k+1)^2 (S(n) is NOT an integer sequence). Notice that when n approaches +oo, D(n) converges to Catalan's constant (A006752). A294970(n) is equal to the numerator of S(n) (when reduced). Therefore B(n)/A(n) = A294970(n) A294971(n) is equal to the denominator of S(n) (when reduced). Therefore C(n)/A(n) = A294971(n). This sequence was used to study the expression B(n)/C(n) (which equals S(n)) in an attempt to find out if Catalan's constant is irrational. LINKS FORMULA Explicit formula: a(n) = gcd( ((2*n+1)!!)^2 * (Sum_{i=0..n}((-1)^i)/(2*i+1)^2), ((2*n+1)!!)^2 ) / ( (((2*n+1)!!)^2) / ( lcm{1,3,5,...,2*n+1} ) )^2. A few relations: gcd(A275286(n), A001818(n+1)) = a(n)*A025549(n+1)^2 = A(n); A275286(n)/A(n) = A294970(n); A001818(n+1)/A(n) = A294971(n); Lim_{n->+oo) A294970(n)/A294971(n) = G (Catalan's Constant, decimal expansion: A006257). EXAMPLE For n = 5: B(n) = A275286(5) = 98607816; C(n) = A001818(5+1) = 108056025; gcd(98607816,108056025) = A(5) = 9; A025549(5+1)^2 = 3^2 = 9; So a(5) = A(5)/A025549(5+1)^2 = 9/9 = 1. MATHEMATICA a[n_] := GCD[(2n+1)!!^2 * Sum[(-1)^k/(2k+1)^2, {k, 0, n}], (2n+1)!!^2]* LCM @@ Range[1, 2n+1, 2]^2 / ((2n+1)!!)^2; Array[a, 100, 0] (* Amiram Eldar, Nov 16 2018 *) PROG (PARI) dfo(n) = (2*n)! / n! / 2^n; a6(n) = dfo(n+1)^2*sum(k=0, n, (-1)^k/(2*k+1)^2); a8(n) = ((2*n)!/(n!*2^n))^2; a9(n) = (((2*n)!/n!)/2^n)/lcm(vector(n, i, 2*i-1)); a(n) = gcd(a6(n) , a8(n+1)) / a9(n+1)^2; \\ Michel Marcus, Nov 08 2018 CROSSREFS Cf. A275286, A001818, A025549, A294970, A294971, A006257. Sequence in context: A045538 A084066 A231472 * A112122 A290856 A010850 Adjacent sequences:  A319147 A319148 A319149 * A319151 A319152 A319153 KEYWORD nonn AUTHOR Tristan Cam, Nov 08 2018 STATUS approved

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Last modified May 14 02:56 EDT 2021. Contains 343871 sequences. (Running on oeis4.)