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A003434 Number of iterations of phi(x) at n needed to reach 1.
(Formerly M0244)
46
0, 1, 2, 2, 3, 2, 3, 3, 3, 3, 4, 3, 4, 3, 4, 4, 5, 3, 4, 4, 4, 4, 5, 4, 5, 4, 4, 4, 5, 4, 5, 5, 5, 5, 5, 4, 5, 4, 5, 5, 6, 4, 5, 5, 5, 5, 6, 5, 5, 5, 6, 5, 6, 4, 6, 5, 5, 5, 6, 5, 6, 5, 5, 6, 6, 5, 6, 6, 6, 5, 6, 5, 6, 5, 6, 5, 6, 5, 6, 6, 5, 6, 7, 5, 7, 5, 6, 6, 7, 5, 6, 6, 6, 6, 6, 6, 7, 5, 6, 6, 7, 6, 7, 6, 6 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,3

COMMENTS

Each number n>1 occurs for the first time at the position A007755(n+1) and for the last time at 2*3^(n-1). - Ivan Neretin, Mar 24 2015

REFERENCES

N. J. A. Sloane and Simon Plouffe, The Encyclopedia of Integer Sequences, Academic Press, 1995 (includes this sequence).

M. V. Subbarao, On a function connected with phi(n), J. Madras Univ. B. 27 (1957), pp. 327-333.

LINKS

T. D. Noe, Table of n, a(n) for n = 1..10000

Hartosh Singh Bal, Gaurav Bhatnagar, Prime number conjectures from the Shapiro class structure, arXiv:1903.09619 [math.NT], 2019.

C. Defant, On Arithmetic Functions Related to Iterates of the Schemmel Totient Functions, J. Int. Seq. 18 (2015) # 15.2.1.

Paul Erdős, Andrew Granville, Carl Pomerance and Claudia Spiro, On the normal behavior of the iterates of some arithmetic functions, Analytic number theory, Birkhäuser Boston, 1990, pp. 165-204.

Paul Erdős, Andrew Granville, Carl Pomerance and Claudia Spiro, On the normal behavior of the iterates of some arithmetic functions, Analytic number theory, Birkhäuser Boston, 1990, pp. 165-204. [Annotated copy with A-numbers]

I. Niven, The iteration of certain arithmetic functions, Canad. J. Math. 2 (1950), pp. 406-408.

H. N. Shapiro, On the iterates of a certain class of arithmetic functions, Comm. Pure Appl. Math. 3 (1950), pp. 259-272.

S. Sivasankaranarayana Pillai, On a function connected with phi(n), Bull. Amer. Math. Soc., 35:6 (1929), pp. 837-841.

S. Sivasankaranarayana Pillai, On a function connected with phi(n), Bull. Amer. Math. Soc., 35.6 (1929), 837-841. (Annotated scanned copy)

FORMULA

a(n) = A049108(n) - 1.

By the definition of a(n) we have for n >= 2 the recursion a(n) = a(phi(n)) + 1. - Ahmed Fares (ahmedfares(AT)my-deja.com), Apr 20 2001

Pillai proved that log(n/2)/log(3) + 1 <= a(n) <= log(n)/log(2) + 1. - Charles R Greathouse IV, Mar 22 2012

EXAMPLE

If n=164 the trajectory is {164,80,32,16,8,4,2,1}. Its length is 8, thus a(164)=7.

MAPLE

A003434 := proc(n)

    local a, e;

    e := n ;

    a :=0 ;

    while e > 1 do

        a := a+1 ;

        e := numtheory[phi](e) ;

    end do:

    a;

end proc:

seq(A003434(n), n=1..40) ; # R. J. Mathar, Jan 09 2017

MATHEMATICA

f[n_] := Length@ NestWhileList[ EulerPhi, n, # != 1 &] - 1; Array[f, 105] (* Robert G. Wilson v, Feb 07 2012 *)

PROG

(PARI) A003434(n)=for(k=0, n, n>1 || return(k); n=eulerphi(n)) /* Works because the loop limits are evaluated only once. Using while(...) takes 50% more time. */ \\ M. F. Hasler, Jul 01 2009

(Haskell)

a003434 n = fst $ until ((== 1) . snd)

                        (\(i, x) -> (i + 1, a000010 x)) (0, n)

-- Reinhard Zumkeller, Feb 08 2013, Jul 03 2011

(Python)

from sympy import totient

def A003434(n):

    c, m = 0, n

    while m > 1:

        c += 1

        m = totient(m)

    return c # Chai Wah Wu, Nov 14 2021

CROSSREFS

Cf. A000010, A007755.

Sequence in context: A235613 A322418 A019569 * A330808 A097849 A100678

Adjacent sequences:  A003431 A003432 A003433 * A003435 A003436 A003437

KEYWORD

nonn,easy,nice,look

AUTHOR

N. J. A. Sloane

STATUS

approved

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Last modified July 1 10:21 EDT 2022. Contains 354959 sequences. (Running on oeis4.)