A241481 -id:A241481 - cp's OEIS frontend

A380303 Numbers m such that GCD of the solutions x to sigma(x) = sigma(m) are setwise coprime.

1, 6, 10, 11, 14, 15, 16, 17, 20, 21, 23, 24, 25, 26, 28, 30, 31, 33, 34, 35, 38, 39, 40, 41, 42, 44, 46, 47, 51, 52, 53, 54, 55, 56, 57, 58, 59, 60, 62, 63, 65, 66, 69, 70, 71, 74, 76, 77, 78, 79, 83, 84, 85, 86, 87, 88, 89, 90, 92, 93, 94, 95, 96, 97, 99, 102, 103, 105, 107, 108, 110, 111, 112, 113, 114, 115, 117, 118, 119, 120, 123, 124, 125, 126, 127, 130, 131, 132, 135, 136, 138, 139, 140, 141, 142, 143, 145, 147, 150

Offset: 1

Max Alekseyev, Jan 19 2025

nonn

Amiram Eldar, Table of n, a(n) for n = 1..10000
Max Alekseyev, PARI/GP Scripts for Miscellaneous Math Problems (invphi.gp).

Cf. A206036, A211656, A241480, A241481, A241625, A380304.

isok(k) = if(k == 1, 1, my(v = invsigma(sigma(k))); #v > 1 && gcd(v) == 1); \\ Amiram Eldar, May 28 2025, using Max Alekseyev's invphi.gp (see links).

Union of {1} and the set difference of A206036 and A241481.

A380304 a(n) = sigma(A380303(n)).

Original entry on oeis.org

1, 12, 18, 12, 24, 24, 31, 18, 42, 32, 24, 60, 31, 42, 56, 72, 32, 48, 54, 48, 60, 56, 90, 42, 96, 84, 72, 48, 72, 98, 54, 120, 72, 120, 80, 90, 60, 168, 96, 104, 84, 144, 96, 144, 72, 114, 140, 96, 168, 80, 84, 224, 108, 132, 120, 180, 90, 234, 168, 128, 144, 120, 252, 98, 156, 216, 104, 192, 108, 280, 216, 152, 248, 114, 240, 144, 182, 180, 144, 360, 168, 224, 156, 312, 128, 252, 132, 336, 240, 270, 288, 140, 336, 192

Offset: 1

Max Alekseyev, Jan 19 2025

nonn

Except for a(1)=1, subsequence of A206421.

Cf. A000203, A206036, A211656, A241480, A241481, A241625, A380303.

a(n) = sigma(A380303(n)) = A000203(A380303(n)).

cp's OEIS Frontend

A380303 Numbers m such that GCD of the solutions x to sigma(x) = sigma(m) are setwise coprime.

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