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A120453 Numbers such that 2*UnitaryPhi(2*UnitaryPhi(n)) = n. 0
2, 4, 6, 12, 16, 30, 48, 60, 168, 240, 256, 510, 768, 1020, 3840, 4080, 14880, 65280, 65536, 131070, 196608, 262140, 983040, 1048560, 16711680, 16776960, 4294901760, 4294967296, 7608944640, 8589934590, 12884901888, 17179869180 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

COMMENTS

Numbers of the form 2^(2^m), 1<=m<=5 exist on the sequence. In other form, it is Product_{0<=i<=m} F_i + 1, 0<=m<=4, where F_i is Fermat prime 2^(2^i)+1

a(33) > 2^35. - Donovan Johnson, May 06 2013

LINKS

Table of n, a(n) for n=1..32.

MAPLE

A047994 := proc(n) p := ifactors(n)[2] ; mul(op(1, op(i, p))^op(2, op(i, p))-1, i=1..nops(p)) ; end proc:

for m from 2 by 2 do if 2*A047994(2*A047994(m)) = m then print(m); end if; end do:

CROSSREFS

Cf. A047994, A117820.

Sequence in context: A220219 A284456 A233968 * A326438 A023995 A018189

Adjacent sequences:  A120450 A120451 A120452 * A120454 A120455 A120456

KEYWORD

nonn

AUTHOR

Yasutoshi Kohmoto, Jul 20 2006

EXTENSIONS

a(21)-a(32) from Donovan Johnson, May 06 2013

STATUS

approved

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Last modified August 12 17:17 EDT 2020. Contains 336439 sequences. (Running on oeis4.)