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%I #26 Nov 15 2022 17:51:47
%S 1,5391411025,
%T 5164037398437051798923642083026622326955987448536772329145127064375
%N Least number k coprime to 2 and 3 such that sigma(k)/k >= n.
%C Data copied from the Hi.gher. Space link where Mercurial, the Spectre calculated the terms. We have a(2) = 5^2*7*...*29 and a(3) = 5^4*7^3*11^2*13^2*17*...*157 ~ 5.16404*10^66. a(4) = 5^5*7^4*11^3*13^3*17^2*19^2*23^2*29^2*31^2*37^2*41*...*853 ~ 1.83947*10^370 is too large to display.
%H Jianing Song, <a href="/A358412/b358412.txt">Table of n, a(n) for n = 1..4</a>
%H Kevin A. Broughan and Qizhi Zhou, <a href="https://hdl.handle.net/10289/1796">Odd multiperfect numbers of abundancy 4</a>, author’s version, Research Commons.
%H Kevin A. Broughan and Qizhi Zhou, <a href="https://doi.org/10.1016/j.jnt.2007.02.001">Odd multiperfect numbers of abundancy 4</a>, Journal of Number Theory 128 (2008) 1566-1575.
%H Mercurial, the Spectre, <a href="http://hi.gher.space/forum/viewtopic.php?f=11&t=2248&sid=cbf9e6743a4ccdcd6cbcadcdf56946db">Abundant numbers coprime to n</a>, Hi.gher. Space.
%e a(2) = A047802(2) = 5391411025 is the smallest abundant number coprime to 2 and 3.
%e Even if there is a number k coprime to 2 and 3 with sigma(k)/k = 3, we have that k is a square since sigma(k) is odd. If omega(k) = m, then 3 = sigma(k)/k < Product_{i=3..m+2} (prime(i)/(prime(i)-1)) => m >= 33, and we have k >= prime(3)^2*...*prime(35)^2 ~ 6.18502*10^112 > A358413(2) ~ 5.16403*10^66. So a(3) = A358413(2).
%e Even if there is a number k coprime to 2 and 3 with sigma(k)/k = 4, there can be at most 2 odd exponents in the prime factorization of k (see Theorem 2.1 of the Broughan and Zhou link). If omega(k) = m, then 4 = sigma(k)/k < Product_{i=3..m+2} (prime(i)/(prime(i)-1)) => m >= 140, and we have k >= prime(3)^2*...*prime(140)^2*prime(141)*prime(142) ~ 2.65585*10^669 > A358414(2) ~ 1.83947*10^370. So a(4) = A358414(2).
%Y Smallest k-abundant number which is not divisible by any of the first n primes: A047802 (k=2), A358413 (k=3), A358414 (k=4).
%Y Least p-rough number k such that sigma(k)/k >= n: A023199 (p=2), A119240 (p=3), this sequence (p=5), A358418 (p=7), A358419 (p=11).
%K nonn,bref,hard
%O 1,2
%A _Jianing Song_, Nov 14 2022