How many combinations are there for a four digit number from 1 to 9? Any array between 1 and 9 into a 4-digit, any combination, how many combinations? How to calculate? And so on, 5-bit, 6-bit How to calculate?

Using permutation and combination to solve the problem
There are 9 numbers from 1 to 9, 4 of which are arranged
That is, A4 9 (9 at the bottom, 4 at the top) = 3024
If it's five digits, it's A5 9 = 15120
If it's 6 digits, it's A6 9 = 60480
If it's 7 digits, it's A7 9 = 181440
If it's 8 digits, a8 9 = 362880
If it's 9 digits, it's A9 9 = 362880
LS is wrong. The numbers can't be repeated. They are arranged in A4 9. Now they are represented by A
If it can be reused, it can't be calculated simply by permutation or combination

What are the four digit permutations of 0-9
Which permutation can be formed by randomly extracting four numbers from the ten numbers of 0,1,2,3,4,5,6,7,8,9? Detailed permutation is needed, not simple number

4536

The 6-digit permutation with 0,1,2,3,4,5,6,7,8,9 should be all permutations
All have to write out, know a lot of you can help me find on the Internet

There are nine options to exclude 0 from the first digit
There are 10 choices for the second digit
The third digit has 10 choices
The fourth digit has 10 choices
The fifth digit has 10 choices
The sixth digit has 10 choices
In this way, 9 * 10 * 10 * 10 * 10 * 10 is calculated according to the multiplication principle
So the answer is 900000

How many combinations are there for a four digit number from 1 to 9? Any array between 1 and 9 into a 4-digit, any combination, how many combinations? How to calculate? And so on, 5-bit, 6-bit How to calculate?