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Math Help - Counting number of distinct digits in natural numbers less than 1,000,000

  1. #1
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    Counting number of distinct digits in natural numbers less than 1,000,000



    I need to do this for k=2,3 only. Any help? Thanks.
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  2. #2
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    Quote Originally Posted by qtpipi View Post


    I need to do this for k=2,3 only. Any help? Thanks.
    Let us do k=2 and you try k=3. First pick 2 digits numbers that you are going to use. There are {9\choose 2} ways of doing that. But once you have choose these two numbers there are \frac{6!}{3!\cdot 3!} ways of ordering them. Multiply those two together and you get your answer.
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  3. #3
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    I have the exact same question, but I have to do it for all 6 values of k. I understand why there are {9\choose2} ways to pick 2 digits, but why are there \frac{6!}{3!\cdot 3!} ways to order them? I've been trying to work the problem out this way:
    For each value of k, treat the number of possible numbers as a 6-digit base-k number, subtract the numbers that don't have k distinct digits, and finally multiply it by the number of possible values for each distinct digit. So, for example, if k=4, I'd work it out as:
    {9\choose4} [(4^6 - 1) - (3^6 - 1) - (2^6 - 1) - 4] = 415800
    The number of numbers in a base 4, 6-digit number, minus all the numbers with only 3 distinct digits, 2 distinct digits, and 1 distinct digit, and multiply it by the number of ways to choose those 4 digits.
    I know my reasoning is wrong, because when I try the same idea for k=5, I get a value over 1 million, which is obviously impossible.

    So how do you calculate the number of ways to order the digits? I get that 6! is because it's a 6-digit number, but what the denominator?

    Thanks.
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