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A120453
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Numbers such that 2*UnitaryPhi(2*UnitaryPhi(n)) = n.
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0
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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
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OFFSET
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1,1
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COMMENTS
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Numbers of the form 2^(2^m), 1 <= m <= 5 are terms.
Numbers of the form Product_{i=0..m} (F_i + 1), 0 <= m <= 4, where F_i is Fermat prime 2^(2^i) + 1 are also terms.
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LINKS
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MAPLE
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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:
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CROSSREFS
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KEYWORD
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nonn
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AUTHOR
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EXTENSIONS
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STATUS
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approved
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