A371414 Euler phi function applied to the cubefull exponentially odd numbers (A335988).

1, 4, 18, 16, 100, 64, 72, 162, 294, 256, 288, 400, 1210, 648, 1024, 1458, 2028, 1176, 2500, 1800, 1152, 1600, 4624, 6498, 2592, 4096, 5292, 4840, 4704, 11638, 4608, 6400, 14406, 5832, 8112, 13122, 23548, 10000, 7200, 28830, 16200, 10368, 16384, 21780, 18496, 19360

Offset: 1

Author

Amiram Eldar, Mar 22 2024

Keywords

• nonn
• easy

Links

• Amiram Eldar, Table of n, a(n) for n = 1..10000

Crossrefs

Cf. A000010, A098198, A335988.

Similar sequences: A323333, A371414, A371415.

Programs

Mathematica

Join[{1}, EulerPhi /@ Select[Range[20000], AllTrue[Last /@ FactorInteger[#], #1 > 1 && OddQ[#1] &] &]]

PARI

lista(max) = {my(f, ans); print1(1, ", "); for(k = 2, max, f = factor(k); ans = 1; for (i = 1, #f~, if (f[i, 2] == 1 || !(f[i, 2] % 2), ans = 0; break)); if(ans, print1(eulerphi(f), ", ")));}

Formula

a(n) = A000010(A335988(n)).

Sum_{n>=1} 1/a(n) = Product_{p prime} (1 + 1/((p-1)^2*(p+1))) = zeta(2)^2 * Product_{p prime} (1 - 2/p^2 + 1/p^3 + 2/p^4) = 1.43921640806700099050... .