“Arithmetic number”的意思、由来-开放百科全书

词条

Arithmetic number

释义

Density
Notes
References

In number theory, an arithmetic number is an integer for which the average of its positive divisors is also an integer. For instance, 6 is an arithmetic number because the average of its divisors is

which is also an integer. However, 2 is not an arithmetic number because its only divisors are 1 and 2, and their average 3/2 is not an integer.

The first numbers in the sequence of arithmetic numbers are

1, 3, 5, 6, 7, 11, 13, 14, 15, 17, 19, 20, 21, 22, 23, 27, 29, 30, 31, 33, 35, 37, 38, 39, 41, 42, 43, 44, 45, 46, 47, 49, ... {{OEIS|id=A003601}}.

Density

It is known that the natural density of such numbers is 1:^[1] indeed, the proportion of numbers less than X which are not arithmetic is asymptotically^[2]

where c = 2{{radic|log 2}} + o(1).

A number N is arithmetic if the number of divisors d(N) divides the sum of divisors σ(N). It is known that the density of integers N obeying the stronger condition that d(N)² divides σ(N) is 1/2.^[1]^[2]

Notes

1. ^¹Guy (2004) p.76
2. ^¹{{cite book | zbl=0478.10027 | last1=Bateman | first1=Paul T. | author1-link=Paul T. Bateman | last2=Erdős | first2=Paul | author2-link=Paul Erdős | last3=Pomerance | first3=Carl | author3-link=Carl Pomerance | last4=Straus | first4=E.G. | author4-link=Ernst G. Straus | chapter=The arithmetic mean of the divisors of an integer| title=Analytic number theory, Proc. Conf., Temple Univ., 1980 | editor-first=M.I. | editor-last=Knopp | editor-link=Marvin Knopp | series=Lecture Notes in Mathematics | volume=899 | pages=197–220 | year=1981 | publisher=Springer-Verlag | url = http://www.math.dartmouth.edu/~carlp/PDF/31.pdf}}

References

{{cite book |last=Guy |first=Richard K. |authorlink=Richard K. Guy |title=Unsolved problems in number theory |publisher=Springer-Verlag |edition=3rd |year=2004 |isbn=978-0-387-20860-2 |zbl=1058.11001 |at=B2}}

2 : Divisor function|Integer sequences

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