Maths Olympiad Prep

Library / /150 of 520

Number theory Difficulty 6.0 National olympiad Find the answer

Determine all natural numbers m,nm, n and all prime numbers pp such that

m(4m2+m+12)=3(pn1) m\left(4 m^{2}+m+12\right)=3\left(p^{n}-1\right)

A number or a short expression. Fractions can be typed as 3/2, and spacing doesn't matter.

Solution

We rewrite the equation in the form

3pn=4m3+m2+12m+3=(4m+1)(m2+3) 3 p^{n}=4 m^{3}+m^{2}+12 m+3=(4 m+1)\left(m^{2}+3\right)

From this, we deduce that there exist natural numbers u,v,a,bu, v, a, b such that 4m+1=3upa4 m+1=3^{u} p^{a} and m2+3=3vpbm^{2}+3=3^{v} p^{b} with u+v=1u+v=1 and a+b=na+b=n.
If m=0m=0, the equation becomes 3pn=33 p^{n}=3, so n=0n=0 and any prime number works.
Now suppose m1m \geqslant 1. Then 4m+1>34 m+1>3 and m2+3>3m^{2}+3>3, so a1a \geqslant 1 and b1b \geqslant 1.
We have

16×3vpb=16(m2+3)=(4m)2+48=((4m+1)1)2+48=32up2a2×3upa+49 \begin{aligned} 16 \times 3^{v} p^{b} & =16\left(m^{2}+3\right)=(4 m)^{2}+48=((4 m+1)-1)^{2}+48 \\ & =3^{2 u} p^{2 a}-2 \times 3^{u} p^{a}+49 \end{aligned}

Since pp divides the left-hand side, it divides the right-hand side, so p49p \mid 49, which gives p=7p=7. We then replace 49 with p2p^{2}:

16×3vpb=32up2a2×3upa+p2 16 \times 3^{v} p^{b}=3^{2 u} p^{2 a}-2 \times 3^{u} p^{a}+p^{2}

If a=1a=1 then 4m+14 m+1 is 7 or 21. Since mm is an integer, we have 4m+1=214 m+1=21, or m=5m=5, which implies 3vpb=m2+3=283^{v} p^{b}=m^{2}+3=28. Impossible. Therefore, a2a \geqslant 2. We deduce that 32up2a2×3upa+p23^{2 u} p^{2 a}-2 \times 3^{u} p^{a}+p^{2} is divisible by p2p^{2}, so 16×3vpb16 \times 3^{v} p^{b} is divisible by p2p^{2}, hence b2b \geqslant 2.
If a3a \geqslant 3, then 16×3vpb=32up2a2×3upa+p216 \times 3^{v} p^{b}=3^{2 u} p^{2 a}-2 \times 3^{u} p^{a}+p^{2} is congruent to p2p^{2} modulo p3p^{3}, so it is not divisible by p3p^{3}, therefore b=2b=2. By dividing by p2p^{2}, we get that 16×3v1[p]16 \times 3^{v} \equiv 1[p]. For v=0v=0 this gives 161[7]16 \equiv 1[7] and for v=1v=1 this gives 481[7]48 \equiv 1[7]. Impossible.
Therefore, we must have a=2a=2 and 4m+1=3u×494 m+1=3^{u} \times 49. If u=0u=0 then m=12m=12 and n=4n=4. If u=1u=1 then 4m+1=1474 m+1=147, which is impossible.
Conclusion: the only solutions are

- m=0,n=0,pm=0, n=0, p any prime, and
- m=12,n=4,p=7m=12, n=4, p=7.

Want a route through all this instead of an archive? The track puts 2,000 problems in a working order, from AMC 10 level to the IMO shortlist.

Source: NuminaMath-1.5, licensed Apache-2.0. Statement and solution reproduced as published; topic and difficulty added by this site.