Talk:06-240/Homework Assignment 4

Divisibility by Prime Number

Pls correct me if I were wrong. The operation of cut away the unit digit is a distraction. If we consider the unit digit, the operation basically is a deduction of a number, and that number is divisible by 7. The whole operation is shown as follow:

86415
105	105/7=21
----
8631
21	21/7=3
---
861
21	21/7=3
--
84
84	84/7=12
-
0	0/7=0

Since it is an operation of series subtraction by multiples of 7, therefore the number we started from is divisible by 7 iff the resulting number is divisible by 7.

Moreover, there is a relationship between the unit digit, 2, and 7. The unit digit multiple by 21(7 $\times$ 3) is equal to the combination of the unit digit with its 2-time as the tenth/hundredth digit.

Unit digit, $x$	$x\times 3\times 7$
0	0
1	21
2	42
3	63
4	84
5	105
6	126
7	147
8	168
9	189

From the table above, I've induced the criterion for divisibility by 17 that is similar operation but the unit digit multiplies by 5 instead of 2. For divisibility by 13, the unit digit multiple by 9. Alright, I think it will be more fun if it's explained by other people. Wongpak 09:38, 5 October 2006 (EDT)

Excellent!

--Drorbn 12:09, 5 October 2006 (EDT)

I found the problem solved three different ways at [1] He mentions the problem as stated can be found in The Dictionary of Curious and Interesting Numbers\ by David Wells.

I prefer his second method. Beginning from the right most digit, multiply corresponding digits by the following sequence of coefficients 1 3 2 6 4 5. For larger numbers sequence repeats. Alternatively the sequence 1 3 2 -1 -3 -2 could be used.

Using the given example $86415\Rightarrow 1(5)+3(1)+2(4)-1(6)-3(8)=-14\Rightarrow 7|86415$

Coefficients can be determined by taking multiplying the previous coefficient by 10 and take the modulus of this new number.

For n=17, sequence begins with 1 multiply by 10 then $10\equiv 10\mod (17),\quad 10\cdot 10\equiv 15\mod (17)\Rightarrow 15\equiv -2\mod (17)$

So for n=17, sequence is: 1, 10, 15, 14, 4, 6, 9, 5, 16, 7, 2, 3, 13, 11, 8, 12

or alternatively: 1, -7, -2, -3, 4, 6, -8, 5, -1, 7, 2, 3, -4, -6, 8, -5