login
This site is supported by donations to The OEIS Foundation.

 

Logo


Hints
(Greetings from The On-Line Encyclopedia of Integer Sequences!)
A271936 Commutative powers: numbers of the form a^b = b^a. 0
1, 4, 16, 27, 256, 3125, 46656, 823543, 16777216, 387420489, 10000000000, 285311670611, 8916100448256, 302875106592253, 11112006825558016, 437893890380859375, 18446744073709551616, 827240261886336764177, 39346408075296537575424, 1978419655660313589123979, 104857600000000000000000000 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,2

COMMENTS

The only integer solutions (a,b) to the equation a^b = b^a are (k,k) for k >= 1, (2,4), and (4,2).

Suppose a<b. If a^b = b^a, log(a^b) = log(b^a) so log(a)/a = log(b)/b.

Now f(x) = log(x)/x is strictly increasing at (0,e) and strictly decreasing at (e, +infinity), so if 0<x<y<e or e<y<x then f(x)<f(y).

To have f(a) = f(b) (which means a^b = b^a), a must be in (0,e) and b in (e, +infinity). Let g(a) be the restriction of f to (0,e) and h(b) to (e, +infinity). Since g and h are monotonic, there is a natural bijection between a and b such that g(a) = h(b).

In (0,e) there are two integers: 1 and 2.

1 won't work since there are no solutions 1^b=b^1 for b>e.

2 will because 2^4 = 4^2 and since we have a bijection this is the only solution.

LINKS

Table of n, a(n) for n=1..21.

FORMULA

a(n+1) = A000302(n), n=0, n=1, n=2;

a(n+1) = A000312(n), n>2.

EXAMPLE

4 is in the list because 2^2 = 2^2;

16 is in the list because 2^4 = 4^2.

MATHEMATICA

Union[Power @@@ Select[Tuples[Range@ 20, 2], Power @@ # == Power @@ Reverse@ # &]] (* Michael De Vlieger, Apr 17 2016 *)

CROSSREFS

Sequence in context: A067688 A097374 A257309 * A046358 A046366 A227609

Adjacent sequences:  A271933 A271934 A271935 * A271937 A271938 A271939

KEYWORD

nonn,easy

AUTHOR

Natan Arie' Consigli, Apr 16 2016

STATUS

approved

Lookup | Welcome | Wiki | Register | Music | Plot 2 | Demos | Index | Browse | More | WebCam
Contribute new seq. or comment | Format | Style Sheet | Transforms | Superseeker | Recent
The OEIS Community | Maintained by The OEIS Foundation Inc.

License Agreements, Terms of Use, Privacy Policy. .

Last modified July 22 15:00 EDT 2019. Contains 325224 sequences. (Running on oeis4.)