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This is a front-end for the Online Encyclopedia of Integer Sequences, made by Christian Perfect. The idea is to provide OEIS entries in non-ancient HTML, and then to think about how they're presented visually. The source code is on GitHub.

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A145680 a(n) = smallest number m such that UnitarySigma(m) = nm/(n-1).

Original entry on oeis.org

6, 2, 3, 4, 5, 216, 7, 8, 9, 5292000, 11, 10584000, 13, 4991499040640000, 165375, 16, 17, 235270656, 19, 101867327360000, 8107610881081625211441398582431641600000, 19235716742537891017605454376709022095843377283072000000, 23, 552063590295800832, 25
Offset: 2

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Author

Yasutoshi Kohmoto Mar 09 2009

Keywords

Comments

If n=p^i+1 then a(n)=p^i.
Unitary sigma = A034448 = sum of divisors d with gcd(d,m/d)=1.
Note that a(27) > 10^70. - Jack Brennen, Feb 04 2014

Examples

			UnitarySigma(216) = 2^2*3^2*7 = (7/6)*216.

Crossrefs

Cf. A034448.
Related sequences include A145681 (and A144672, A144673, A144674) for 3/2-URPMS, and A144949-A144951 for 5/4-URPMS. - M. F. Hasler, Jan 29 2014

Extensions

Corrected a(13) and added a(22) found by Jack Brennen. M. F. Hasler, Feb 04 2014
a(23)-a(26) from Jack Brennen, Feb 04 2014
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