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 A073759 Largest number that is neither a divisor of nor relatively prime to n, or 0 if no such number exists. 6
 0, 0, 0, 0, 0, 4, 0, 6, 6, 8, 0, 10, 0, 12, 12, 14, 0, 16, 0, 18, 18, 20, 0, 22, 20, 24, 24, 26, 0, 28, 0, 30, 30, 32, 30, 34, 0, 36, 36, 38, 0, 40, 0, 42, 42, 44, 0, 46, 42, 48, 48, 50, 0, 52, 50, 54, 54, 56, 0, 58, 0, 60, 60, 62, 60, 64, 0, 66, 66, 68, 0, 70, 0, 72, 72, 74, 70, 76 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,6 COMMENTS Largest "unrelated" number to n. From Michael De Vlieger, Mar 28 2016 (Start): Primes n have no unrelated numbers m < n since all such numbers are coprime to n. Unrelated numbers m must be composite since primes must either divide or be coprime to n. m = 1 is not counted as unrelated as it divides and is coprime to n. a(4) = 0 since 4 is the smallest composite and unrelated numbers m with respect to n must be composite and smaller than n. All other composite n have at least one unrelated number m. The test for unrelated numbers m that belong to n is 1 < gcd(m, n) < m. (End) LINKS Michael De Vlieger, Table of n, a(n) for n = 1..10000 EXAMPLE n = 20: unrelated set to 20 = {6,8,12,14,15,16,18},largest is a(20) = 18. MATHEMATICA tn[x_] := Table[w, {w, 1, x}]; di[x_] := Divisors[x]; dr[x_] := Union[di[x], rrs[x]]; rrs[x_] := Flatten[Position[GCD[tn[x], x], 1]]; unr[x_]: =Complement[tn[x], dr[x]]; Table[Max[unr[w]], {w, 1, 128}]; (for empty sets 0 was used). Table[t = Select[r = Range[n - 1], Divisible[n, #] || GCD[n, #] == 1 &]; Max[Join[{0}, Complement[r, t]]], {n, 78}] (* Jayanta Basu, Jul 09 2013 *) Table[SelectFirst[Range[n - 2, 2, -1], 1 < GCD[#, n] < # &] /. n_ /; MissingQ@ n -> 0, {n, 100}] (* Michael De Vlieger, Mar 28 2016, Version 10.2 *) PROG (PARI) a(n) = {forstep(k=n-2, 1, -1, if ((gcd(n, k) != 1) && (n % k), return (k)); ); 0; } \\ Michel Marcus, Mar 29 2016 CROSSREFS Cf. A045763, A073758. Sequence in context: A073758 A133995 A019629 * A066760 A102393 A228131 Adjacent sequences:  A073756 A073757 A073758 * A073760 A073761 A073762 KEYWORD nonn AUTHOR Labos Elemer, Aug 15 2002 STATUS approved

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Last modified July 5 01:22 EDT 2020. Contains 335457 sequences. (Running on oeis4.)