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A063830 a(n+1) is the smallest odd m whose cototient equals a(n). 0
1, 3, 9, 21, 45, 117, 297, 585, 1521, 3105, 6993, 14553, 43653, 90885, 185925, 397125, 847125, 1813125, 3238725, 7556829, 17253789, 36910365, 94571997, 220301277, 475043037, 47979336637, 183450404605, 525019294077 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,2

LINKS

Table of n, a(n) for n=1..28.

FORMULA

a(n) = Min{x, odd : A051953[ a(n-1) ]=a(n)}; a(1)=1; a(2)=3, least odd prime; a(n) = Min[ Select[ Range[ a, b ], Equal[ #-EulerPhi[ # ], a(n-1) ]& ] ].

EXAMPLE

a(5)=45, cototient(45) = 45 - Phi(45) = 45 - 24 = 21 = a(4). This iteration with even numbers might stop, like {1,2,4,6,10} does if the last computed number has no inverse cototient, like 10 which is a non-cototient number.

CROSSREFS

Cf. A005278, A051953.

Sequence in context: A068156 A166452 A052101 * A062444 A141156 A262197

Adjacent sequences:  A063827 A063828 A063829 * A063831 A063832 A063833

KEYWORD

nonn

AUTHOR

Labos Elemer, Aug 21 2001

EXTENSIONS

More terms from David Wasserman, Jul 23 2002

a(26)-a(28) from Donovan Johnson, Feb 06 2010

STATUS

approved

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Last modified February 28 09:40 EST 2020. Contains 332323 sequences. (Running on oeis4.)