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A173746 Numbers n such that tau(tau(n))= rad(n). 1
1, 2, 4, 16, 27, 64, 72, 96, 108, 288, 432, 486, 648, 768, 972, 1024, 1536, 1728, 3456, 4096, 5832, 6561, 13122, 17496, 20736, 24576, 27648, 39366, 41472, 65536, 98304, 104976, 110592, 147456, 186624, 256000, 262144, 314928, 400000, 419904, 472392 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,2

COMMENTS

Tau = A000005 is the number of divisors of its argument. rad(n) = A007947(n) is the product of the primes dividing n.

Note that rad() is idempotent: rad(rad(n)) = rad(n). - R. J. Mathar, Nov 07 2011

LINKS

Donovan Johnson, Table of n, a(n) for n = 1..496 (terms < 10^18)

FORMULA

{n : A010553(n) = A007947(n)}.

EXAMPLE

288 is in the sequence because tau(288)= 18, tau(18)=6, rad(288)=6.

MAPLE

A010553 := proc(n)

        numtheory[tau](numtheory[tau](n)) ;

end proc:

for n from 1 to 480000 do

        if A010553(n) = A007947(n) then

                printf("%d, ", n) ;

        end if;

end do: # R. J. Mathar, Nov 07 2011

CROSSREFS

Sequence in context: A104258 A143904 A144797 * A095803 A032464 A171381

Adjacent sequences:  A173743 A173744 A173745 * A173747 A173748 A173749

KEYWORD

nonn

AUTHOR

Michel Lagneau, Feb 23 2010

EXTENSIONS

Example corrected and edited by Michel Lagneau, Apr 25 2010

STATUS

approved

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Last modified July 22 18:10 EDT 2017. Contains 289671 sequences.