x x power floor x x ?

How many positive integers α \alpha less than 1000, are there such that the equation x x = α \large x^{\lfloor x \rfloor} = \alpha has a solution for x x ?


The answer is 412.

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1 solution

Guilherme Niedu
Mar 8, 2016

If x = 1, then α \alpha = 1

If 2 \leq x < 3, the exponent will be 2, so:

  • x = 4 \sqrt 4 = 2 \rightarrow α \alpha = 4

  • x = 5 \sqrt 5 \rightarrow α \alpha = 5

  • x = 6 \sqrt 6 \rightarrow α \alpha = 6

  • x = 7 \sqrt 7 \rightarrow α \alpha = 7

  • x = 8 \sqrt 8 \rightarrow α \alpha = 8

We stop at 8 \sqrt 8 because \lfloor 9 \sqrt 9 \rfloor = 3, which changes the exponent. So:

If 3 \leq x < 4, the exponent will be 3, so:

  • x = 2 3 7 \sqrt[3]27 = 3 \rightarrow α \alpha = 27

  • x = 2 3 8 \sqrt[3]28 \rightarrow α \alpha = 28

\vdots

  • x = 6 3 3 \sqrt[3]63 \rightarrow α \alpha = 63

Again, for x = 6 3 4 \sqrt[3]64 the exponent changes, so:

If 4 \leq x < 5, the exponent will be 4, so:

  • x = 2 4 56 \sqrt[4]256 = 4 \rightarrow α \alpha = 256

  • x = 2 4 57 \sqrt[4]257 \rightarrow α \alpha = 257

\vdots

  • x = 6 4 24 \sqrt[4]624 \rightarrow α \alpha = 624

Again, for x = 6 4 25 \sqrt[4]625 the exponent changes, so:

If 5 \leq x < 6, the exponent will be 5. But 5 5 5^5 = 3125 > 1000.

So, α \alpha can only be {1, 4, 5, 6, 7, 8, 27, 28, 29, ... ,63, 256, 257, 258,.... 624}, which gives 412 possible values

Exactly Same Way.

Kushagra Sahni - 5 years, 3 months ago

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