A181840 - OEIS

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A181840

Maximum of { k>0 | k<n and k is strongly prime to n }, or zero if this set is empty.

0, 1, 0, 0, 0, 3, 0, 5, 5, 7, 7, 9, 7, 11, 11, 13, 13, 15, 13, 17, 17, 19, 19, 21, 19, 23, 23, 25, 25, 27, 23, 29, 29, 31, 31, 33, 31, 35, 35, 37, 37, 39, 37, 41, 41, 43, 43, 45, 43, 47, 47, 49, 49, 51, 49, 53, 53, 55, 55, 57, 53, 59, 59, 61, 61, 63, 61, 65, 65, 67, 67, 69

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OFFSET

0,6

COMMENTS

k is strongly prime to n iff k is relatively prime to n and k does not divide n-1.

Editor's note: It seems that 1 is strongly prime to 1 by convention.

LINKS

Table of n, a(n) for n=0..71.

Peter Luschny, Strong coprimality.

EXAMPLE

a(11) = max{3, 4, 6, 7, 8, 9} = 9.

MAPLE

with(numtheory):

Primes := n -> select(k->isprime(k), {$1..n}):

StrongCoprimes := n -> select(k->igcd(k, n)=1, {$1..n}) minus divisors(n-1):

A181840 := proc(n) max(op(StrongCoprimes(n))); subs(infinity=0, %) end:

MATHEMATICA

a[n_] := Max[ Select[ Range[n-1], CoprimeQ[#, n] && ! Divisible[n-1, #] &]] /. -Infinity -> 0; a[1] = 1; Table[a[n], {n, 0, 71}] (* Jean-François Alcover, Jun 27 2013 *)

PROG

(PARI) a(n)={ forstep(k=n-2, 2, -1, gcd(k, n)==1 & (n-1)%k & return(k)); n==1 } \\ M. F. Hasler, Nov 17 2010

CROSSREFS

Cf. A181830, A181839.

Sequence in context: A354836 A197690 A351692 * A198432 A102391 A038556

Adjacent sequences: A181837 A181838 A181839 * A181841 A181842 A181843

KEYWORD

nonn

AUTHOR

Peter Luschny, Nov 17 2010

STATUS

approved