OFFSET
1,1
COMMENTS
Let p(n) = number of unitary divisors k of n, k<n, that are relatively prime to n. Let p_1(n) = p(n), p_r(n) = p(p_{r-1}(n)). Sequence gives minimal r such that p_r(n)=1.
REFERENCES
M. Lal, H. Wareham and R. Mifflin, Iterates of the bi-unitary totient function, Utilitas Math., 10 (1976), 347-350.
N. J. A. Sloane and Simon Plouffe, The Encyclopedia of Integer Sequences, Academic Press, 1995 (includes this sequence).
LINKS
Amiram Eldar, Table of n, a(n) for n = 1..97 (terms 1..78 from Donovan Johnson)
MAPLE
L := [seq(0, i=0..100)] ;
for n from 1 do
itr := A225320(n) ;
if itr < nops(L) then
if op(itr, L) = 0 then
L := subsop(itr=n, L) ;
print(L) ;
end if;
end if;
end do: # R. J. Mathar, May 02 2013
MATHEMATICA
A116550[1] = 1; A116550[n_] := With[{pp = Power @@@ FactorInteger[n]}, Count[Range[n], m_ /; Intersection[pp, Power @@@ FactorInteger[m]] == {}]]; A225320[n_] := A225320[n] = If[n == 1, 0, 1+A225320[A116550[n]]]; L = Array[0&, 100]; For[n = 1, n <= 40000, n++, itr = A225320[n]; If[itr < Length[L], If[L[[itr]] == 0, L = ReplacePart[L, itr -> n]; Print[Select[L, Positive] // Last]]]]; Select[L, Positive] (* Jean-François Alcover, Jan 13 2014, after R. J. Mathar *)
CROSSREFS
KEYWORD
nonn,nice
AUTHOR
STATUS
approved