# Any special class of numbers other than 2^n for which Collatz is true?

The title says it all. Are there any special class of numbers other than $2^n$ for which Collatz is true? (I mean numbers such as other powers? or some other formula that is in essence not a manipulation of $2^n$. )

Thanks.

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All numbers less than $20 × 2^{58}$, if that counts... –  Daniel Freedman Dec 29 '11 at 12:47
Trivially, $2^nk$ for any fixed $k$ known to satisfy the conjecture. –  Ilmari Karonen Dec 29 '11 at 12:53