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 A119584 a(n) = Sum_{k=1..phi(n)-1} t(n,k)*t(n,k+1), where t(n,k) is the k-th positive integer which is coprime to n and phi(n) is the number of positive integers which are <= n and are coprime to n. 0
 0, 0, 2, 3, 20, 5, 70, 53, 121, 87, 330, 117, 572, 305, 507, 553, 1360, 481, 1938, 873, 1586, 1405, 3542, 1241, 3846, 2415, 4006, 2765, 7308, 1875, 8990, 4945, 6828, 5675, 9333, 4525, 15540, 8053, 11567, 7745, 21320, 6047, 24682, 12005, 15244, 14625 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,3 COMMENTS All primes are records and there exists records which are not primes, but they are rare (see A120033). - Robert G. Wilson v, Jun 05 2006 LINKS Table of n, a(n) for n=1..46. EXAMPLE The positive integers which are <= 8 and are coprime to 8 are 1, 3, 5 and 7. So a(8) = 1*3 + 3*5 + 5*7 = 53. MATHEMATICA a[n_] := Block[{s = Select[ Range@n, GCD[ #, n] == 1 &]}, Plus @@ (Most@s*Rest@s)]; Array[a, 46] (* Robert G. Wilson v, Jun 05 2006 *) PROG (PARI) a(n) = my(v=select(x->gcd(x, n)==1, [1..n])); sum(k=1, #v-1, v[k]*v[k+1]); \\ Michel Marcus, Mar 07 2024 CROSSREFS Sequence in context: A038584 A108022 A108884 * A344546 A279719 A279672 Adjacent sequences: A119581 A119582 A119583 * A119585 A119586 A119587 KEYWORD nonn AUTHOR Leroy Quet, May 31 2006 EXTENSIONS More terms from Robert G. Wilson v, Jun 05 2006 STATUS approved

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Last modified May 27 14:41 EDT 2024. Contains 372861 sequences. (Running on oeis4.)