A336698 -id:A336698 - cp's OEIS frontend

A336844 a(n) = A336698(A003961(n)).

1, 1, 1, 7, 1, 1, 1, 3, 1, 1, 1, 5, 5, 1, 1, 61, 3, 1, 1, 7, 5, 1, 1, 1, 29, 5, 5, 5, 1, 1, 5, 23, 11, 3, 1, 101, 11, 1, 7, 3, 3, 5, 1, 23, 1, 1, 7, 91, 67, 29, 1, 59, 1, 5, 1, 1, 5, 1, 1, 5, 9, 5, 47, 547, 5, 11, 5, 33, 23, 1, 19, 39, 3, 11, 43, 5, 11, 7, 11, 61, 391, 3, 23, 59, 3, 1, 1, 9, 25, 1, 7, 49, 29, 7, 1, 137

Offset: 1

Antti Karttunen, Aug 06 2020

nonn

Antti Karttunen, Table of n, a(n) for n = 1..16383
Antti Karttunen, Data supplement: n, a(n) computed for n = 1..65537
Index entries for sequences computed from indices in prime factorization
Index entries for sequences related to sigma(n)

Cf. A000265, A000203, A003961, A003973, A336698, A336699, A336842.

A000265(n) = (n>>valuation(n,2));
A003973(n) = { my(f = factor(n)); for(i=1, #f~, f[i, 1] = nextprime(f[i, 1]+1)); sigma(factorback(f)); };
A336844(n) = A000265(1+A000265(A003973(n)));

a(n) = A000265(1+A000265(A003973(n))).

a(n) = A336698(A003961(n)) = A336699(A003961(n)).

A161942 Odd part of sum of divisors of n.

Original entry on oeis.org

1, 3, 1, 7, 3, 3, 1, 15, 13, 9, 3, 7, 7, 3, 3, 31, 9, 39, 5, 21, 1, 9, 3, 15, 31, 21, 5, 7, 15, 9, 1, 63, 3, 27, 3, 91, 19, 15, 7, 45, 21, 3, 11, 21, 39, 9, 3, 31, 57, 93, 9, 49, 27, 15, 9, 15, 5, 45, 15, 21, 31, 3, 13, 127, 21, 9, 17, 63, 3, 9, 9, 195, 37, 57, 31, 35, 3, 21, 5, 93, 121, 63

Offset: 1

Franklin T. Adams-Watters, Jun 22 2009

It is conjectured that iteration of this function will always reach 1. This implies the nonexistence of odd perfect numbers. This is equivalent to the same question for A000593, which can be expressed as the sum of the divisors of the odd part of n.
Up to 20000000, there are only two odd numbers with a(n) and a(a(n)) both >= n: 81 and 18966025. See A162284.
For the nonexistence proof of odd perfect numbers, it is enough to show that this sequence has no fixed points beyond the initial one. This is equivalent to a similar condition given for A326042. - Antti Karttunen, Jun 17 2019

Cf. A000265, A000203, A000593, A162284, A326042, A336698. A348992, A351544.
Cf. A348741, A348742, A348743.
One less than A337194.

oddPart[n_] := n/2^IntegerExponent[n, 2]; a[n_] := oddPart[ DivisorSigma[1, n]]; Table[a[n], {n, 1, 82}] (* Jean-François Alcover, Sep 03 2012 *)

oddpart(n)=n/2^valuation(n,2);
a(n)=oddpart(sigma(n));

from sympy import divisor_sigma
def A161942(n): return (m:=int(divisor_sigma(n)))>>(~m&m-1).bit_length() # Chai Wah Wu, Mar 17 2023

(define (A161942 n) (A000265 (A000203 n))) ;; [For the implementations of A000203 and A000265, see under the respective entries]. - Antti Karttunen, Nov 18 2017

Multiplicative with a(p^e) = oddpart((p^{e+1}-1)/(p-1)), where oddpart(n) = A000265(n) is the largest odd divisor of n.
a(n) = A000265(A000203(n)).
a(n) = A337194(n)-1. - Antti Karttunen, Nov 30 2024

A337194 a(n) = 1 + A000265(sigma(n)), where A000265 gives the odd part.

Original entry on oeis.org

2, 4, 2, 8, 4, 4, 2, 16, 14, 10, 4, 8, 8, 4, 4, 32, 10, 40, 6, 22, 2, 10, 4, 16, 32, 22, 6, 8, 16, 10, 2, 64, 4, 28, 4, 92, 20, 16, 8, 46, 22, 4, 12, 22, 40, 10, 4, 32, 58, 94, 10, 50, 28, 16, 10, 16, 6, 46, 16, 22, 32, 4, 14, 128, 22, 10, 18, 64, 4, 10, 10, 196, 38, 58, 32, 36, 4, 22, 6, 94, 122, 64, 22, 8, 28, 34

Offset: 1

Antti Karttunen, Aug 18 2020

nonn

Antti Karttunen, Table of n, a(n) for n = 1..65537
Index entries for sequences related to sigma(n)

One more than A161942.

Cf. A000203, A000265, A336698, A337195, A337196.

A000265(n) = (n>>valuation(n,2));
A337194(n) = (1+A000265(sigma(n)));

a(n) = 1+A000265(A000203(n)).
a(n) = A336698(n) * 2^A337195(n).
A007949(a(n)) = A337196(n).

A336700 Numbers k such that the odd part of (1+k) divides (1 + odd part of sigma(k)).

Original entry on oeis.org

1, 3, 7, 15, 31, 63, 127, 255, 511, 1023, 2047, 2431, 2943, 3775, 4095, 8191, 13311, 14335, 16383, 17407, 21951, 22527, 32767, 34335, 44031, 57855, 65535, 85375, 131071, 204799, 262143, 376831, 524287, 923647, 1048575, 1562623, 1632255, 2056191, 2097151, 2744319, 4194303, 6815743, 8388607, 8781823, 10059775, 16777215

Offset: 1

Antti Karttunen, Aug 02 2020

nonn

Numbers k for which A337194(k) = 1+A161942(k) is a multiple of A000265(1+k).

Conjecture: After 1, all terms are of the form 4u+3 (in A004767). If this could be proved, then it would refute at once the existence of both the odd perfect numbers and the quasiperfect numbers. Concentrating on the latter is probably easier, as it is known that all quasiperfect numbers must be odd squares, thus k is of the form 4u+1, in which case the condition given in A336701 that A000265(1+A000265(sigma(k))) must be equal to A000265(1+k) reduces to a simpler form, A000265(1+sigma(k)) = (1+k)/2, and as k = s^2, with s odd, so (s^2 + 1)/2 should divide 1+sigma(s^2). Does that condition allow any other solutions than s=1 ? See A337339.

Antti Karttunen, Table of n, a(n) for n = 1..77; all terms < 2^32
Paolo Cattaneo, Sui numeri quasiperfetti, Bollettino dell’Unione Matematica Italiana, Serie 3, Vol.6(1951), n.1, p. 59-62.
P. Hagis and G. L. Cohen, Some Results Concerning Quasiperfect Numbers, J. Austral. Math. Soc. Ser. A 33, 275-286, 1982.
V. Siva Rama Prasad and C. Sunitha, On quasiperfect numbers, Notes on Number Theory and Discrete Mathematics, Vol. 23, 2017, No. 3, 73-78.
Eric Weisstein's World of Mathematics, Quasiperfect Number
Index entries for sequences where odd perfect numbers must occur, if they exist at all
Index entries for sequences related to sigma(n)

Cf. A000203, A000265, A004767, A016754, A161942, A228058, A336697, A336698, A337339, A337342.

Subsequences: A000225, A336701 (terms where the quotient is a power of 2).

Block[{f}, f[n_] := n/2^IntegerExponent[n, 2]; Select[Range[2^20], Mod[f[1 + f[DivisorSigma[1, #]]], f[1 + #]] == 0 &] ] (* Michael De Vlieger, Aug 22 2020 *)

A000265(n) = (n>>valuation(n,2));
isA336700(n) = !((1+A000265(sigma(n)))%A000265(1+n));

A336701 Numbers k for which A000265(1+A000265(sigma(k))) is equal to A000265(1+k).

Original entry on oeis.org

1, 3, 7, 15, 31, 127, 1023, 8191, 34335, 57855, 131071, 524287, 2147483647

Offset: 1

Antti Karttunen, Aug 02 2020

Numbers k such that A336698(k) [= A000265(1+A161942(k))] is equal to A000265(1+k).
Numbers k such that A337194(k) = 2^e * A000265(1+k), for some e >= 1, where that e = A337195(k).
Any odd perfect number would trivially satisfy this condition.
Also, all hypothetical quasiperfect numbers, numbers k that satisfy sigma(k) = 2k+1, would be members.
Question: Is A066175 a subsequence of this sequence?
From Antti Karttunen, Aug 23 2020: (Start)
Numbers k such that (1+k) = 2^e * A336698(k), for some e >= 0.
Thus numbers k such that for some e >= 0, (1+k) = 2^(e-A337195(k)) * A337194(k), or equally, that A337194(k) = 2^(A337195(k)-e) * (1+k).
Conjecture: There are no even terms. This is equivalent to claim that there are no k such that A336698(k) = 1+k: If we assume that k is even, then in above equations we set e=0, and the requirement will then become that A337194(k) = 2^A337195(k)*(1+k), thus 1+k = A336698(k) = A000265(1+A000265(sigma(k))).
(End)

Paolo Cattaneo, Sui numeri quasiperfetti, Bollettino dell’Unione Matematica Italiana, Serie 3, Vol.6(1951), n.1, p. 59-62.
P. Hagis and G. L. Cohen, Some Results Concerning Quasiperfect Numbers, J. Austral. Math. Soc. Ser. A 33, 275-286, 1982.
V. Siva Rama Prasad and C. Sunitha, On quasiperfect numbers, Notes on Number Theory and Discrete Mathematics, Vol. 23, 2017, No. 3, 73-78.
Eric Weisstein's World of Mathematics, Quasiperfect Number
Index entries for sequences where odd perfect numbers must occur, if they exist at all
Index entries for sequences related to sigma(n)

Subsequence of A336700.
Cf. A000203, A000265, A066175, A161942, A336698, A336699, A336937, A337194, A337195.
Cf. A000668 (a subsequence).
See also comments in A326042, A332223.

Block[{f}, f[n_] := n/2^IntegerExponent[n, 2]; Select[Range[2^20], f[1 + f[DivisorSigma[1, #]]] == f[1 + #] &] ] (* Michael De Vlieger, Aug 22 2020 *)

A000265(n)  = (n>>valuation(n,2));
isA336701(n) = (A000265(1+A000265(sigma(n))) == A000265(1+n));

A336699 a(n) = A000265(1+A000265(sigma(A000265(n)))), where A000265(k) gives the odd part of k, and sigma is the sum of divisors function.

Original entry on oeis.org

1, 1, 1, 1, 1, 1, 1, 1, 7, 1, 1, 1, 1, 1, 1, 1, 5, 7, 3, 1, 1, 1, 1, 1, 1, 1, 3, 1, 1, 1, 1, 1, 1, 5, 1, 7, 5, 3, 1, 1, 11, 1, 3, 1, 5, 1, 1, 1, 29, 1, 5, 1, 7, 3, 5, 1, 3, 1, 1, 1, 1, 1, 7, 1, 11, 1, 9, 5, 1, 1, 5, 7, 19, 5, 1, 3, 1, 1, 3, 1, 61, 11, 11, 1, 7, 3, 1, 1, 23, 5, 1, 1, 1, 1, 1, 1, 25, 29, 5, 1, 13, 5, 7, 1, 1

Offset: 1

Antti Karttunen, Aug 02 2020

See the "lacunae" in the scatter plot. - Antti Karttunen, Mar 27 2022

Antti Karttunen, Table of n, a(n) for n = 1..16384
Antti Karttunen, Data supplement: n, a(n) computed for n = 1..65537
Jon Maiga, Computer-generated formulas for A336699, Sequence Machine.
Index entries for sequences related to sigma(n)

Cf. A000203, A000265, A000593, A002131, A082903, A161942, A171435, A322250, A336698, A336700, A336701, A336844 [= a(A003961(n))], A351565.

A000265(n) = (n>>valuation(n,2));
A336699(n) = A000265(1+A000265(sigma(A000265(n))));

a(n) = A000265(1+A000265(A000593(n))) = A000265(1+A161942(A000265(n))).
a(n) = A336698(A000265(n)).
From Antti Karttunen, Mar 27 2022: (Start)
a(n) = A351565(A000593(n)).
[The following formulas were discovered by Sequence Machine]:
a(n) = A351565(A002131(n)) = A000265(1+A000265(A002131(n))).
a(n) = A336698(1+A322250(n)).
a(n) = A171435(A000593(n)+A082903(n)).
(End)

A337195 The 2-adic valuation of 1+A000265(sigma(n)), where A000265 gives the odd part.

Original entry on oeis.org

1, 2, 1, 3, 2, 2, 1, 4, 1, 1, 2, 3, 3, 2, 2, 5, 1, 3, 1, 1, 1, 1, 2, 4, 5, 1, 1, 3, 4, 1, 1, 6, 2, 2, 2, 2, 2, 4, 3, 1, 1, 2, 2, 1, 3, 1, 2, 5, 1, 1, 1, 1, 2, 4, 1, 4, 1, 1, 4, 1, 5, 2, 1, 7, 1, 1, 1, 6, 2, 1, 1, 2, 1, 1, 5, 2, 2, 1, 1, 1, 1, 6, 1, 3, 2, 1, 4, 1, 1, 1, 3, 1, 1, 1, 4, 6, 1, 2, 3, 1, 2, 2, 1, 1, 2

Offset: 1

Antti Karttunen, Aug 18 2020

nonn

Antti Karttunen, Table of n, a(n) for n = 1..65537
Index entries for sequences related to sigma(n)

Cf. A000203, A000265, A007814, A161942, A336698, A337194, A337196.

A007814(n) = valuation(n,2);
A000265(n) = (n>>A007814(n));
A337195(n) = A007814(1+A000265(sigma(n)));

a(n) = A007814(A337194(n)) = A007814(1+A000265(A000203(n))).

A351565 Odd part of Kimberling's paraphrases: a(n) = A000265(A003602(n)).

Original entry on oeis.org

1, 1, 1, 1, 3, 1, 1, 1, 5, 3, 3, 1, 7, 1, 1, 1, 9, 5, 5, 3, 11, 3, 3, 1, 13, 7, 7, 1, 15, 1, 1, 1, 17, 9, 9, 5, 19, 5, 5, 3, 21, 11, 11, 3, 23, 3, 3, 1, 25, 13, 13, 7, 27, 7, 7, 1, 29, 15, 15, 1, 31, 1, 1, 1, 33, 17, 17, 9, 35, 9, 9, 5, 37, 19, 19, 5, 39, 5, 5, 3, 41, 21, 21, 11, 43, 11, 11, 3, 45, 23, 23, 3, 47

Offset: 1

Antti Karttunen, Mar 27 2022

nonn

Antti Karttunen, Table of n, a(n) for n = 1..16384
Antti Karttunen, Data supplement: n, a(n) computed for n = 1..65537
Index entries for sequences related to binary expansion of n

Cf. A000265, A003602, A023758 (gives the positions of 1's after its initial zero-term).

Cf. also A336698, A336699.

A000265(n) = (n>>valuation(n,2));
A351565(n) = A000265(1+A000265(n));

def A351565(n): return (m:=1+(n>>(~n&n-1).bit_length()))>>(~m&m-1).bit_length()  # Chai Wah Wu, Jan 03 2024

a(n) = A000265(A003602(n)) = A000265(1+A000265(n)).

A337196 The 3-adic valuation of 1+A000265(sigma(n)), where A000265 gives the odd part.

Original entry on oeis.org

0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 1, 0, 0, 0, 0, 0, 0, 0, 1, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 1, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 1, 0, 0, 0, 0, 0, 0, 0, 0, 0, 2, 0, 0, 0, 0, 0, 0, 0, 0, 2, 0, 0, 1, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0

Offset: 1

Antti Karttunen, Aug 18 2020

nonn

Antti Karttunen, Table of n, a(n) for n = 1..65537
Index entries for sequences related to sigma(n)

Cf. A000203, A000265, A007949, A074941, A161942, A336698, A336700, A337194, A337195.

Cf. A337197 (the first occurrence of each n).

Array[IntegerExponent[1 + #/2^IntegerExponent[#, 2], 3] &@ DivisorSigma[1, #] &, 105] (* Michael De Vlieger, Feb 22 2021 *)

A007949(n) = valuation(n,3);
A000265(n) = (n>>valuation(n,2));
A337196(n) = A007949(1+A000265(sigma(n)));

a(n) = A007949(A337194(n)) = A007949(1+A000265(A000203(n))).

a(n) = A007949(A336698(n)).

A337198 Number of distinct prime factors in A337194(n) = 1+A000265(sigma(n)), where A000265(k) gives the odd part of k.

Original entry on oeis.org

1, 1, 1, 1, 1, 1, 1, 1, 2, 2, 1, 1, 1, 1, 1, 1, 2, 2, 2, 2, 1, 2, 1, 1, 1, 2, 2, 1, 1, 2, 1, 1, 1, 2, 1, 2, 2, 1, 1, 2, 2, 1, 2, 2, 2, 2, 1, 1, 2, 2, 2, 2, 2, 1, 2, 1, 2, 2, 1, 2, 1, 1, 2, 1, 2, 2, 2, 1, 1, 2, 2, 2, 2, 2, 1, 2, 1, 2, 2, 2, 2, 1, 2, 1, 2, 2, 1, 2, 2, 2, 1, 2, 1, 2, 1, 1, 2, 2, 2, 2, 2, 2, 2, 2, 1

Offset: 1

Antti Karttunen, Aug 20 2020

nonn

The first 3 occurs at a(137).

Antti Karttunen, Table of n, a(n) for n = 1..65537
Index entries for sequences related to sigma(n)

Cf. A000203, A000265, A001221, A161942, A336698, A337194.

Cf. also A058062, A336691, A337199.

A000265(n) = (n>>valuation(n,2));
A337198(n) = (omega(1+A000265(sigma(n))));

a(n) = A001221(A337194(n)) = 1 + A001221(A336698(n)).

cp's OEIS Frontend

A336844 a(n) = A336698(A003961(n)).

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A161942 Odd part of sum of divisors of n.

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A337194 a(n) = 1 + A000265(sigma(n)), where A000265 gives the odd part.

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A336700 Numbers k such that the odd part of (1+k) divides (1 + odd part of sigma(k)).

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A336701 Numbers k for which A000265(1+A000265(sigma(k))) is equal to A000265(1+k).

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A336699 a(n) = A000265(1+A000265(sigma(A000265(n)))), where A000265(k) gives the odd part of k, and sigma is the sum of divisors function.

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A337195 The 2-adic valuation of 1+A000265(sigma(n)), where A000265 gives the odd part.

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A351565 Odd part of Kimberling's paraphrases: a(n) = A000265(A003602(n)).

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