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 Which multiple of 9? (Posted on 2010-09-17)
It is a well known fact that if you permute the digits of a number the difference will be a multiple of 9.

Define the sequence D, where D(n) is the smallest positive value that can be increased by 9n through a permutation of its digits. No leading zeroes are allowed so the first term is D(1)=12 not 10

1) Find the next 14 terms of D.

2) Note D(8) is the greatest n with two digits. What is the greatest n with 3, 4, 5, ... digits?

3) There are some numbers a, b such that a≠b but D(a)=D(b). Prove there are infinitely many such pairs.

4) Sometimes D(n)>9n and sometimes D(n)<9n. Prove that both cases happen an infinity of times.
5) Are there any values of n such that D(n)=9n?

 No Solution Yet Submitted by Jer Rating: 4.0000 (1 votes)

Comments: ( Back to comment list | You must be logged in to post comments.)
 Part 3: Still guessing. Spoiler? | Comment 6 of 8 |
109= D(89) = D(88)
1099 = D(979) = D(978)
10099 = D(9889) = D(9888)
100999 = D(99789) = D(99788)

This is not all pairs a, b such that a≠b but D(a)=D(b), but it is enough to demonstrate that there is an infinite number of them.

 Posted by Steve Herman on 2010-09-18 15:17:19

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