OFFSET
1,1
COMMENTS
In each case, 2 is the fixed point that is reached (1 is the other fixed point of the x -> A181819(x) map).
Includes all integers whose prime signature a) contains two or more distinct numbers, and b) contains no number that occurs the same number of times as any other number. The first member of this sequence that does not fit that description is 75675600, whose prime signature is (4,3,2,2,1,1).
A full characterization is: Numbers whose prime signature (1) has not all equal multiplicities but (2) the numbers of distinct parts appearing with each distinct multiplicity are all equal. For example, the prime signature of 2520 is {1,1,2,3}, which satisfies (1) but fails (2), as the numbers of distinct parts appearing with each distinct multiplicity are 1 (with multiplicity 2, the part being 1) and 2 (with multiplicity 1, the parts being 2 and 3). Hence the sequence does not contain 2520. - Gus Wiseman, Jan 02 2019
LINKS
Amiram Eldar, Table of n, a(n) for n = 1..10000
Eric Weisstein's World of Mathematics, Fixed Point
Eric Weisstein's World of Mathematics, Map
EXAMPLE
1. 180 requires exactly five iterations under the x -> A181819(x) map to reach a fixed point (namely, 2). A181819(180) = 18; A181819(18) = 6; A181819(6) = 4; A181819(4) = 3; A181819(3) = 2 (and A181819(2) = 2).
2. The prime signature of 180 (2^2*3^2*5) is (2,2,1).
a. Two distinct numbers appear in (2,2,1) (namely, 1 and 2).
b. Neither 1 nor 2 appears in (2,2,1) the same number of times as any other number that appears there.
MATHEMATICA
Select[Range[1000], With[{sig=Sort[Last/@FactorInteger[#]]}, And[!SameQ@@Length/@Split[sig], SameQ@@Length/@Union/@GatherBy[sig, Length[Position[sig, #]]&]]]&] (* Gus Wiseman, Jan 02 2019 *)
CROSSREFS
KEYWORD
nonn
AUTHOR
Matthew Vandermast, Jan 04 2011
STATUS
approved