A351551 - OEIS

A351551

Numbers k such that the largest unitary divisor of sigma(k) that is coprime with A003961(k) is also a unitary divisor of k.

1, 2, 10, 34, 106, 120, 216, 260, 340, 408, 440, 580, 672, 696, 820, 1060, 1272, 1666, 1780, 1940, 2136, 2340, 2464, 3320, 3576, 3960, 4280, 4536, 5280, 5380, 5860, 6456, 6960, 7520, 8746, 8840, 9120, 9632, 10040, 10776, 12528, 12640, 13464, 14560, 16180, 16660, 17400, 17620, 19040, 19416, 19992, 21320, 22176, 22968

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OFFSET

1,2

COMMENTS

Numbers k for which A351546(k) is a unitary divisor of k.

The condition guarantees that A351555(k) = 0, therefore this is a subsequence of A351554.

The condition is also a necessary condition for A349745, therefore it is a subsequence of this sequence.

All six known 3-perfect numbers (A005820) are included in this sequence.

All 65 known 5-multiperfects (A046060) are included in this sequence.

Not all multiperfects (A007691) are present (only 587 of the first 1600 are), but all 23 known terms of A323653 are terms, while none of the (even) terms of A046061 or A336702 are.

LINKS

Antti Karttunen, Table of n, a(n) for n = 1..20223

Index entries for sequences computed from indices in prime factorization

Index entries for sequences related to sigma(n)

EXAMPLE

For n = 672 = 2^5 * 3^1 * 7^1, and the largest unitary divisor of the sigma(672) [= 2^5 * 3^2 * 7^1] coprime with A003961(672) [= 13365 = 3^5 * 5^1 * 11^1] is 2^5 * 7^1 = 224, therefore A351546(672) is a unitary divisor of 672, and 672 is included in this sequence.

PROG

(PARI)

A003961(n) = { my(f = factor(n)); for(i=1, #f~, f[i, 1] = nextprime(f[i, 1]+1)); factorback(f); };

A351546(n) = { my(f=factor(sigma(n)), u=A003961(n)); prod(k=1, #f~, f[k, 1]^((0!=(u%f[k, 1]))*f[k, 2])); };

isA351551(n) = { my(u=A351546(n)); (!(n%u) && 1==gcd(u, n/u)); };

CROSSREFS

Cf. A000203, A000396, A003961, A007691, A046061, A065997, A336702, A351546, A351555, A353633 (characteristic function).

Subsequence of A351552 and of A351554.

Cf. A349745, A351550 (subsequences), A005820, A046060, A323653 (very likely subsequences).

Sequence in context: A036799 A190161 A196969 * A351550 A119193 A124634

Adjacent sequences: A351548 A351549 A351550 * A351552 A351553 A351554

KEYWORD

nonn

AUTHOR

Antti Karttunen, Feb 16 2022

STATUS

approved