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A329821 Largest k for which sigma(k) = A002191(n), where A002191 = range of sigma, the sum-of-divisors function A000203. 1
1, 2, 3, 5, 4, 7, 11, 9, 13, 8, 17, 19, 23, 12, 29, 25, 31, 22, 37, 18, 27, 41, 43, 47, 53, 39, 49, 59, 61, 32, 67, 71, 73, 45, 79, 83, 89, 36, 50, 77, 97, 101, 103, 107, 109, 91, 113, 95, 81, 75, 82, 64, 127, 131, 121, 137, 139, 119, 149, 151, 125 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,2

LINKS

Table of n, a(n) for n=1..61.

M. Alekseyev, PARI/GP Scripts for Miscellaneous Math Problems: invphi.gp, Oct. 2005

FORMULA

a(n) = A085790(m,A054973(m)) with m = A002191(n).

EXAMPLE

The possible values of sigma(x) are A002191 = {1, 3, 4, 6, 7, 8, 12, ...}.

The 7th value is 12 = sigma(x) for x = 6 or 11. Since 11 is the largest such value, a(7) = 11.

PROG

(PARI) A329821(n)=vecmax(invsigma(A002191(n))) \\ see Alekseyev link for invsigma(). An invsigmaMax() function is announced.

CROSSREFS

Cf. A000203 (sigma), A002191 (range of sigma), A085790 (table of pre-images of x in A002191), A054973 (number of solutions of sigma(x) = n).

Cf. A051444 (smallest k such that sigma(k) = n).

Sequence in context: A064620 A064216 A075300 * A259153 A028691 A246353

Adjacent sequences:  A329818 A329819 A329820 * A329822 A329823 A329824

KEYWORD

nonn

AUTHOR

M. F. Hasler, Nov 22 2019

STATUS

approved

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Last modified August 8 23:02 EDT 2020. Contains 336300 sequences. (Running on oeis4.)