A113520 a(n) = largest nonprime nonnegative integer < n which is coprime to n.

0, 1, 1, 1, 4, 1, 6, 1, 8, 9, 10, 1, 12, 9, 14, 15, 16, 1, 18, 9, 20, 21, 22, 1, 24, 25, 26, 27, 28, 1, 30, 27, 32, 33, 34, 35, 36, 35, 38, 39, 40, 25, 42, 39, 44, 45, 46, 35, 48, 49, 50, 51, 52, 49, 54, 55, 56, 57, 58, 49, 60, 57, 62, 63, 64, 65, 66, 65, 68, 69, 70, 65, 72, 69, 74

Offset

1,5

Comments

a(1) = 0 because 0 is coprime with 1, for our purposes and is < 1.

The numbers n with a(n) = 1 are given in A048597. - Stefan Steinerberger, Oct 16 2007

Links

Diana Mecum, Table of n, a(n) for n = 1..1163 [From Diana L. Mecum, Aug 05 2008]

Example

a(14) = 9 because 9 is the largest nonprime integer < 14 which is coprime to 14 (since the other nonprime integers > 9 and < 14 {10 and 12} aren't coprime with 14).

Mathematica

a = {0}; For[n = 2, n < 70, n++, i = n - 1; While[PrimeQ[i] || GCD[n, i] > 1, i-- ]; AppendTo[a, i]]; a (* Stefan Steinerberger, Oct 16 2007 *)
lnp[n_]:=Module[{k=n-1}, While[PrimeQ[k]||!CoprimeQ[k, n], k--]; k]; Array[ lnp, 80] (* Harvey P. Dale, May 12 2019 *)

Crossrefs

Sequence in context: A354433 A005451 A135683 * A232597 A197008 A344442

Adjacent sequences: A113517 A113518 A113519 * A113521 A113522 A113523

Keyword

nonn

Author

Leroy Quet, Jan 12 2006

Extensions

More terms from Stefan Steinerberger, Oct 16 2007

Terms 70 through 1163 from Diana L. Mecum, Aug 05 2008

Status

approved