A349745 - OEIS

A349745

Numbers k for which k * gcd(sigma(k), A003961(k)) is equal to sigma(k) * gcd(k, A003961(k)), where A003961 shifts the prime factorization one step towards larger primes, and sigma is the sum of divisors function.

1, 120, 216, 672, 2464, 22176, 228480, 523776, 640640, 837760, 5581440, 5765760, 7539840, 12999168, 19603584, 33860736, 38342304, 71344000, 95472000, 102136320, 197308800, 220093440, 345080736, 459818240, 807009280, 975576960, 1476304896, 1510831360, 1773584640

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OFFSET

1,2

COMMENTS

Numbers k for which k * A342671(k) = A000203(k) * A322361(k).

Numbers k such that gcd(A064987(k), A191002(k)) = gcd(A064987(k), A341529(k)).

Obviously, all odd terms in this sequence must be squares.

All the terms k of A005820 that satisfy A007949(k) < A007814(k) [i.e., whose 3-adic valuation is strictly less than their 2-adic valuation] are also terms of this sequence. Incidentally, the first six known terms of A005820 satisfy this condition, while on the other hand, any hypothetical odd 3-perfect number would be excluded from this sequence. Also, as a corollary, any hypothetical 3-perfect numbers of the form 4u+2 must not be multiples of 3 if they are to appear here. Similarly for any k which occurs in A349169, for 2*k to occur in this sequence, it shouldn't be a multiple of 3 and k should also be a term of A191218. See question 2 and its partial answer in A349169.

From Antti Karttunen, Feb 13-20 2022: (Start)

Question: Are all terms/2 (A351548) abundant, from n > 1 onward?

Note that of the 65 known 5-multiperfect numbers, all others except these three 1245087725796543283200, 1940351499647188992000, 4010059765937523916800 are also included in this sequence. The three exceptions are distinguished by the fact that their 3 and 5-adic valuations are equal. In 62 others the former is larger.

If k satisfying the condition were of the form 4u+2, then it should be one of the terms of A191218 doubled as only then both k and sigma(k) are of the form 4u+2, with equal 2-adic valuations for both. More precisely, one of the terms of A351538.

(End)

LINKS

Amiram Eldar, Table of n, a(n) for n = 1..52 (terms below 10^11)

Antti Karttunen, Ratio A342671(n)/A322361(n) plotted with OEIS Plot2 tool

Index entries for sequences computed from indices in prime factorization

Index entries for sequences related to sigma(n)

FORMULA

For all n >= 1, A007814(A000203(a(n))) = A007814(a(n)). [sigma preserves the 2-adic valuation of the terms of this sequence]

MATHEMATICA

f1[p_, e_] := (p^(e + 1) - 1)/(p - 1); f2[p_, e_] := NextPrime[p]^e; q[1] = True; q[n_] := n * GCD[(s = Times @@ f1 @@@ (f = FactorInteger[n])), (r = Times @@ f2 @@@ f)] == s*GCD[n, r]; Select[Range[10^6], q] (* Amiram Eldar, Nov 29 2021 *)

PROG

(PARI)

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

isA349745(n) = { my(s=sigma(n), u=A003961(n)); (n*gcd(s, u) == (s*gcd(n, u))); };

CROSSREFS

Cf. A000203, A003961, A005101, A005820, A007814, A007949, A046060, A064987, A191002, A191218, A248150, A322361, A341529, A342671, A347870, A351534, A351536, A351538.

Cf. also A349169, A349746, A351458, A351549 for other variants.

Subsequence of A351554 and also of its subsequence A351551.

Cf. A351459 (subsequence, intersection with A351458), A351548 (terms halved).

Sequence in context: A003015 A098565 A084142 * A256814 A232743 A146950

Adjacent sequences: A349742 A349743 A349744 * A349746 A349747 A349748

KEYWORD

nonn

AUTHOR

Antti Karttunen, Nov 29 2021

STATUS

approved