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A367511
Highly composite numbers h(k) = A002182(k) such that h >= rad(h)^2, where rad() = A007947().
1
1, 4, 36, 48, 45360, 50400
OFFSET
1,2
COMMENTS
Alternatively, this sequence lists h(k) such that A301413(k) >= A002110(A108602(k)), where A301413 is the "variable part" v described on page 5 of 12 of the Siano paper.
This sequence is likely finite and full. See Chapter III regarding the structure of "Highly Composite Numbers".
Terms larger than 36 are in A366250; A366250 is in A364702, which is in turn a proper subset of A332785, itself contained in A126706.
36 is in A365308, a proper subset of A303606, contained in A131605, in turn contained in A286708.
LINKS
Srinivasa Ramanujan, Highly Composite Numbers, Proc. London Math. Soc. (1916) Vol. 2, No. 14, 347-409.
D. B. Siano and J. D. Siano, An Algorithm for Generating Highly Composite Numbers, 1994.
EXAMPLE
Let P(n) = A002110(n).
a(1) = h(1) = 1 since 1 >= 1^2.
a(2) = h(3) = 4 since 4 >= P(1)^2, 4 >= 2^2.
a(3) = h(7) = 36 since 36 >= P(2)^2, 36 >= 6^2.
a(4) = h(8) = 48 since 48 >= P(2)^2, 48 >= 6^2.
a(5) = h(26) = 43560 since 43560 >= P(4)^2, where P(4) = 210, and 210^2 = 44100.
a(6) = h(27) = 50400 since 50400 >= P(4)^2.
Let V(i) = A301414(i) and let P(j) = A002110(j).
Plot of highly composite h = V(i)*P(j) at (x,y) = (j,i), i = 1..16, j = 1..7, showing h in this sequence in parentheses, and h in A168263 marked with an asterisk (*):
V(i)\P(j) 1 2 6 30 210 2310 30030 ...
+---------------------------------------
1 |(1*) 2* 6*
2 | (4*) 12* 60*
4 | 24* 120* 840*
6 | (36) 180* 1260*
8 | (48) 240 1680*
12 | 360 2520 27720*
24 | 720 5040 55440 720720
36 | 7560 83160 1081080
48 | 10080 110880 1441440
72 | 15120 166320 2162160
96 | 20160 221760 2882880
120 | 25200 277200 3603600
144 | 332640 4324320
216 | (45360) 498960 6486480
240 | (50400) 554400 7207200
...
MATHEMATICA
(* First load function f at A025487, then run the following: *)
s = Union@ Flatten@ f[12];
t = Map[DivisorSigma[0, #] &, s];
h = Map[s[[FirstPosition[t, #][[1]]]] &, Union@ FoldList[Max, t]];
Reap[Do[If[# >= Product[Prime[j], {j, PrimeNu[#]}]^2, Sow[#]] &[ h[[i]] ],
{i, Length[h]}] ][[-1, 1]]
KEYWORD
nonn,more
AUTHOR
Michael De Vlieger, Feb 08 2024
STATUS
approved