login

Year-end appeal: Please make a donation to the OEIS Foundation to support ongoing development and maintenance of the OEIS. We are now in our 61st year, we have over 378,000 sequences, and we’ve reached 11,000 citations (which often say “discovered thanks to the OEIS”).

A098202
a(n) is the length of the iteration trajectory when the cototient function (A051953) is applied to the n-th primorial number (A002110(n)).
2
3, 5, 8, 12, 18, 20, 31, 32, 41, 43, 61, 65, 80, 77, 95, 125, 131, 125, 157, 173, 140, 192, 195, 221, 213, 212, 261, 269, 277, 300, 296, 321, 336, 329, 358, 367, 379, 405, 428, 439, 438, 464, 477, 493, 506, 454, 491, 542, 564, 588, 543, 600, 639, 660
OFFSET
1,1
FORMULA
a(n) = A053475(A002110(n)). - Robert G. Wilson v, Sep 22 2004
EXAMPLE
For n = 3: list = {30,22,12,8,4,2,1,0}, a(4) = 8.
MATHEMATICA
g[x_] := x - EulerPhi[x]; f[x_] := Length[ FixedPointList[g, x]] - 1; q[x_] := Product[ Prime[j], {j, x}]; Table[ f[ q[n]], {n, 33}]
a[n_] := Length@ NestWhileList[(# - EulerPhi[#])&, Times @@ Prime[Range[n]], # > 0 &]; Array[a, 30] (* Amiram Eldar, Nov 19 2024 *)
PROG
(PARI) a(n) = {my(p = prod(i=1, n, prime(i)), c = 1); while(p > 0, c++; p -= eulerphi(p)); c; } \\ Amiram Eldar, Nov 19 2024
CROSSREFS
KEYWORD
nonn,more
AUTHOR
Labos Elemer, Sep 22 2004
EXTENSIONS
More terms from Robert G. Wilson v, Sep 22 2004
a(37)-a(54) from Amiram Eldar, Nov 19 2024
STATUS
approved