A181836 - OEIS

%I #8 Jun 28 2013 05:07:15

%S 1,1,1,1,1,3,1,5,15,35,7,21,35,385,165,143,1001,15015,5005,85085,

%T 51051,46189,20995,440895,1616615,7436429,1716099,2860165,5311735,

%U 15935205,7436429,215656441,3234846615

%N The product of primes <= n that are strongly prime to n.

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

%H Peter Luschny, <a href="http://www.oeis.org/wiki/User:Peter_Luschny/StrongCoprimality">Strong coprimality</a>.

%e a(11) = 3 * 7 = 21.

%p with(numtheory):

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

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

%p StrongCoprimePrimes := n -> Primes(n) intersect StrongCoprimes(n):

%p A181836 := proc(n) local i; mul(i,i=StrongCoprimePrimes(n)) end:

%t a[n_] := Times @@ Select[Range[2, n], PrimeQ[#] && CoprimeQ[#, n] && !Divisible[n-1, #] &]; Table[a[n], {n, 0, 32}] (* _Jean-François Alcover_, Jun 28 2013 *)

%Y Cf. A181831, A181832, A181833, A181834, A181835, A001783.

%K nonn

%O 0,6

%A _Peter Luschny_, Nov 17 2010