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 A365900 Highly composite numbers k that remain highly composite when recursively divided by squarefree kernel. 2
 1, 2, 4, 6, 12, 24, 36, 60, 120, 180, 360, 720, 840, 1260, 2520, 5040, 7560, 25200, 27720, 55440, 83160, 277200, 720720, 1081080, 3603600, 10810800, 21621600, 61261200, 183783600, 367567200, 3491888400, 6983776800, 48886437600, 73329656400, 80313433200, 160626866400, 1124388064800, 1686582097200, 32607253879200, 48910880818800, 1010824870255200, 1516237305382800 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,2 COMMENTS Let h(n) = A002182(n). Let f(x) = x/rad(x) = A301413(x), where rad(n) = A007947(n) is a primorial and x is in h. If f(h(k)) = m is highly composite, then we apply f(m) until we reach 1 or m that is not highly composite. Let S be the chain of highly composite terms that result from the recursion of f beginning with k in A002182. Terms in S are nondecreasing and each appear in this sequence. Example: beginning with h(51), we have {21621600, 720, 24, 4, 2, 1}. Terms that follow h(51) in the chain appear in this sequence. There are 19 known terms j in S = A301414 = union({A301413}) that are highly composite. f(h(k)) = j is a necessary but insufficient condition for h(k) to appear in this sequence. The numbers j in {48, 240, 10080, 15120, 20160, 50400, 17297280} do not yield terms in this sequence, because {48, 240, 10080, 50400} settle to 8, S(32) = h(22) = 15120 settles to 72, S(33) = h(23) = 20160 ends up at 96, and the largest of the 19 terms, S(62) = h(50) = 17297280 ends up at 576, all of which are not highly composite. It appears that there are only 19 terms that enable membership in this sequence. LINKS Table of n, a(n) for n=1..42. Michael De Vlieger, Plot of h(k) = S(i)*P(omega(h(n))) at (x,y) = (i, omega(h(k)) highlighting k such that h(k) is in this sequence. Achim Flammenkamp, Highly composite numbers. Eric Weisstein's World of Mathematics, Highly Composite Number. EXAMPLE 1 is in this sequence since f(1) = 1 and 1 is highly composite. 2 is in this sequence since f(2) = 1 and 1 is highly composite. 12 is in this sequence since f(12) = 2, and f(2) = 1, both highly composite. 48 is not in this sequence since f(48) = 48/6 = 8, and 8 is not highly composite. Applying f recursively to h(128) = 1516237305382800 yields the following chain: 1516237305382800 -> 7560 -> 36 -> 6 -> 1, all highly composite. It seems that this is the largest term in the sequence. . Tree plot of terms: 1 --- 2 --- 4 --- 24 --- 720 --- 21621600 | | | | |- 367567200 | | | | |- 6983776800 | | | | |_ 160626866400 | | | | | | | |- 5040 -- 48886437600 | | | | |- 1124388064800 | | | | |- 32607253879200 | | | | |_ 1010824870255200 | | | | | | | |- 55440 | | | |_ 720720 | | | | | |- 120 -- 25200 | | | |- 277200 | | | |- 3603600 | | | |_ 61261200 | | | | | |_ 840 | | | |-12 --- 360 -- 10810800 | | | |- 183783600 | | | |- 3491888400 | | | |_ 80313433200 | | | | | |- 2520 | | |_ 27720 | | | |_60 | |_ 6 -- 36 --- 7560 --- 73329656400 | | |- 1686582097200 | | |- 48910880818800 | | |_ 1516237305382800 | | | |_ 83160 -- 1081080 | |-180 |_1260 MATHEMATICA (* Program loads highly composite numbers from A002182 bfile *) a2182 = Import["https://oeis.org/A002182/b002182.txt", "Data"][[All, -1]]; rad[x_] := rad[x] = Times @@ FactorInteger[x][[All, 1]]; Select[Array[ NestWhileList[#/rad[#] &, a2182[[#]], And[# > 1, ! FreeQ[a2182, #]] &] &, 250], Last[#] == 1 &][[All, 1]] CROSSREFS Cf. A002110, A002182, A301413, A301414. Sequence in context: A236021 A340637 A362782 * A365681 A333665 A355595 Adjacent sequences: A365897 A365898 A365899 * A365901 A365902 A365903 KEYWORD nonn,fini,full AUTHOR Michael De Vlieger, Oct 06 2023 STATUS approved

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Last modified July 24 10:37 EDT 2024. Contains 374583 sequences. (Running on oeis4.)