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a(n) = Sum_{k=1..n} tau(gcd(k,n))^(n/gcd(k,n)), where tau(n) is the number of divisors of n.
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%I #18 May 11 2021 10:35:16

%S 1,3,4,9,6,26,8,49,25,140,12,240,14,782,156,1215,18,3349,20,5130,800,

%T 20498,24,19558,151,98324,3148,111492,30,270624,32,551091,20520,

%U 2097176,924,1716189,38,9437210,98348,8630496,42,25362724,44,43714584,266346,184549406,48,137141048,813,671096867

%N a(n) = Sum_{k=1..n} tau(gcd(k,n))^(n/gcd(k,n)), where tau(n) is the number of divisors of n.

%F a(n) = Sum_{d|n} phi(n/d) * tau(d)^(n/d).

%F If p is prime, a(p) = 1 + p.

%t a[n_] := DivisorSum[n, EulerPhi[n/#] * DivisorSigma[0, #]^(n/#) &]; Array[a, 50] (* _Amiram Eldar_, May 11 2021 *)

%o (PARI) a(n) = sum(k=1, n, numdiv(gcd(k, n))^(n/gcd(k, n)));

%o (PARI) a(n) = sumdiv(n, d, eulerphi(n/d)*numdiv(d)^(n/d));

%Y Cf. A000005, A279789, A342424, A344140, A344194.

%K nonn

%O 1,2

%A _Seiichi Manyama_, May 11 2021