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

Author

Leroy Quet, Jan 12 2006

Keywords

• nonn

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

Examples

			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).
		

Links

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

Programs

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 *)

Extensions

More terms from Stefan Steinerberger, Oct 16 2007

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