Re: Combinatoric question


James Waldby wrote:
Proginoskes wrote:
Jules wrote:
Larry Coon wrote:
...
What's the correct approach to problems like, "find
how many n-digit numbers have the digit d at least x
times." E.g., "find how many 10-digit numbers have
at least five sevens."

BTW, if numbers aren't allowed to start with zeros, this is a MUCH
harder problem.

If f(n) is the number of n-digit numbers, leading zero allowed,
with at least five 7's, and g(n) is likewise but leading zero not
allowed, then g(n) = f(n)-f(n-1), so it appears to make little
difference.

Good point; I shouldn't have said "MUCH".

--- Christopher Heckman

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