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A058213 This is a triangle arrangement of solutions of phi(x)=2^n (n>=0), where phi=A000010 is Euler's totient function. Each row corresponds to a particular n and its length is n+2 for 0<=n<=31, 32 for n>=32. (This assumes that there are only 5 Fermat primes.). 5
1, 2, 3, 4, 6, 5, 8, 10, 12, 15, 16, 20, 24, 30, 17, 32, 34, 40, 48, 60, 51, 64, 68, 80, 96, 102, 120, 85, 128, 136, 160, 170, 192, 204, 240, 255, 256, 272, 320, 340, 384, 408, 480, 510, 257, 512, 514, 544, 640, 680, 768, 816, 960, 1020, 771, 1024, 1028, 1088 (list; graph; refs; listen; history; text; internal format)
OFFSET

0,2

COMMENTS

phi(x) is a power of 2 if and only if x is a power of 2 multiplied by a product of distinct Fermat primes. So if, as is conjectured, there are only 5 Fermat primes, then there are only 32 possibilities for the odd part of x, namely the divisors of 2^32-1, given in A004729.

The same numbers, in increasing order, are given in A003401.

The first entry in row n is the n-th divisor of 2^32-1 for 0<=n<=31 (A004729) and is 2^(n+1) for n>=32. The last entry in row n is given in A058215.

LINKS

T. D. Noe, Table of n, a(n) for n = 0..1000

EXAMPLE

Triangle begins:

{1,2},

{3,4,6},

{5,8,10,12},

{15,16,20,24,30},

{17,32,34,40,48,60},

{51,64,68,80,96,102,120},

{85,128,136,160,170,192,204,240},

...

MATHEMATICA

phiinv[ n_, pl_ ] := Module[ {i, p, e, pe, val}, If[ pl=={}, Return[ If[ n==1, {1}, {} ] ] ]; val={}; p=Last[ pl ]; For[ e=0; pe=1, e==0||Mod[ n, (p-1)pe/p ]==0, e++; pe*=p, val=Join[ val, pe*phiinv[ If[ e==0, n, n*p/pe/(p-1) ], Drop[ pl, -1 ] ] ] ]; Sort[ val ] ]; phiinv[ n_ ] := phiinv[ n, Select[ 1+Divisors[ n ], PrimeQ ] ]; Join@@(phiinv[ 2^# ]&/@Range[ 0, 10 ]) (* phiinv[ n, pl ] = list of x with phi(x)=n and all prime divisors of x in list pl. phiinv[ n ] = list of x with phi(x)=n *)

CROSSREFS

Cf. A000010, A001317, A003401, A004729, A019434, A045544, A047999, A053576, A054432, A058214, A058215.

Sequence in context: A080738 A032447 A224531 * A080997 A151942 A054582

Adjacent sequences:  A058210 A058211 A058212 * A058214 A058215 A058216

KEYWORD

nonn,tabf

AUTHOR

Labos Elemer, Nov 30 2000

EXTENSIONS

Edited by Dean Hickerson, Jan 25 2002

STATUS

approved

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Last modified April 26 08:16 EDT 2019. Contains 322472 sequences. (Running on oeis4.)