A346465 - OEIS

%I #17 Oct 05 2021 20:24:48

%S 9,11,18,27,32,36,45,50,53,54,63,68,72,74,78,81,90,95,99,100,108,116,

%T 117,126,127,135,137,144,147,150,153,155,158,162,171,179,180,182,189,

%U 198,200,204,207,216,221,225,233,234,242,243,250,252,261,263,270,279

%N Numbers k such that (4^k - 2)*(4^k - 1)/Clausen(2*k, 1) is not squarefree, where Clausen(n, m) = A160014(n, m).

%C Also numbers k such that 6*GaussBinomial(2*k, 2, 2)/denominator(Bernoulli(2*k, 1)) is not squarefree.

%F The positive multiples of 9 form a subsequence.

%F k is a term if and only if A346463(k) > A007947(A346463(k)).

%p with(NumberTheory): isa := n -> not IsSquareFree(((4^n - 2)*(4^n - 1))/

%p mul(i, i = select(isprime, map(i -> i+1, Divisors(2*n))))):

%p select(isa, [$(1..100)]);

%t q[n_] := Product[k, {k, Select[Table[d + 1, {d, Divisors[2 n]}], PrimeQ]}];

%t isA[n_] := ! SquareFreeQ[((4^n - 2) (4^n -1)) / q[n]];

%t Select[Range[50],  isA]

%Y Cf. A006095, A002445, A007947, A160014, A346463, A346464.

%K nonn

%O 1,1

%A _Peter Luschny_, Jul 20 2021

%E More terms from _Jinyuan Wang_, Jul 23 2021