login
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
OFFSET
1,2
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 * A353266 A259153 A355066
KEYWORD
nonn
AUTHOR
M. F. Hasler, Nov 22 2019
STATUS
approved