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A225821 a(n) = Product_{p | p is prime and p, p-1 both divide n}.

%I A225821 #55 Jul 31 2019 07:27:04
%S A225821 1,2,1,2,1,6,1,2,1,2,1,6,1,2,1,2,1,6,1,10,1,2,1,6,1,2,1,2,1,6,1,2,1,2,
%T A225821 1,6,1,2,1,10,1,42,1,2,1,2,1,6,1,2,1,2,1,6,1,2,1,2,1,30,1,2,1,2,1,6,1,
%U A225821 2,1,2,1,6,1,2,1,2,1,6,1,10,1,2,1,42
%N A225821 a(n) = Product_{p | p is prime and p, p-1 both divide n}.
%C A225821 a(n) = 2 iff n is even and is a term of A226872. - _Daniel Suteu_, Jul 28 2019
%C A225821 From _Bernard Schott_, Jul 30 2019: (Start)
%C A225821 a(n) = n if n = 1, 2, 6, 42, 1806.
%C A225821 a(n) = 6 if n is of the form 2^i*3^j, i and j >= 1, so if n is a term of A033845.
%C A225821 a(n) = 10 if n is of the form 2^i*5^j, i >= 2 and j >= 1.
%C A225821 a(n) = 30 if n is of the form 2^i*3^j*5^k, i >=2, j >= 1 and k >= 1. (End)
%H A225821 Eric M. Schmidt, <a href="/A225821/b225821.txt">Table of n, a(n) for n = 1..10000</a>
%F A225821 a(n) = denominator(A031971(n)/n) = gcd(n, A027642(n)). - _Daniel Suteu_, Jul 28 2019
%t A225821 fa=FactorInteger; d[m_]:= Product[If[IntegerQ[m/(fa[m][[i, 1]]-1)],fa[m][[i, 1]], 1], {i, Length@fa@m}]; Table[d[n], {n, 1, 333}]
%o A225821 (Sage) def A225821(n) : return prod(p for (p,m) in factor(n) if n%(p-1)==0) # _Eric M. Schmidt_, Jul 31 2013
%o A225821 (PARI) a(n)=my(f=factor(n)[,1]); prod(i=1,#f,if(n%(f[i]-1)==0,f[i],1)) \\ _Charles R Greathouse IV_, Nov 13 2013
%Y A225821 Cf. A173557, A027760.
%K A225821 nonn
%O A225821 1,2
%A A225821 _José María Grau Ribas_, Jul 30 2013