Mod m, m+1, and m+2 residues

Number Theory Level pending

The mod m residue of the integer x is 1, the mod m+1 residue of the integer x is 2, and the mod m +2 residue of the integer x is 3. If m>1, and the value of x is minimized, what is m+x?

11 13 6 16 23

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2 solutions

From the given conditions of the problem, for positive integers x , m , p , q , r x, m, p, q, r we can write

x = p m + 1 = q ( m + 1 ) + 2 = r ( m + 2 ) + 3 x=pm+1=q(m+1)+2=r(m+2)+3

Since we are searching for the minimum of x x and since m > 1 m>1 , we take the minimum value of m = 2 m=2 into consideration. Then we have

x = 2 p + 1 = 3 q + 2 = 4 r + 3 x=2p+1=3q+2=4r+3

p = 3 q + 1 2 = 2 r + 1 \implies p=\dfrac {3q+1}{2}=2r+1

So q q must be odd and so also 3 q + 1 2 \dfrac {3q+1}{2}

We can easily see that this is achieved for q = 3 q=3 , when p = 5 p=5 and r = 2 r=2

Then x = 11 x=11 and m + x = 2 + 11 = 13 m+x=2+11=\boxed {13} .

We try to first find x and m:

When x has the residue 1 mod m, the for some integer k 1 k_{1} :

x 1 m \frac{x-1}{m} = k 1 k_{1}

When x has the residue 2 mod m+1, the for some integer k 2 k_{2} :

x 2 m + 1 \frac{x-2}{m+1} = k 2 k_{2}

When x has the residue 3 mod m+2, the for some integer k 3 k_{3} :

x 3 m + 2 \frac{x-3}{m+2} = k 3 k_{3}

Now we can minimize m first.

m has to be greater than 1, so the most minimized value of m is 2.

Plugging in m=2 we get:

x 1 2 \frac{x-1}{2}

and:

x 2 3 \frac{x-2}{3}

and:

x 3 4 \frac{x-3}{4}

Using guess and check, we find that the minimum value of x is 11.

So the answer is 11+2= 13 \boxed{13}

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