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Number theory Difficulty 5.6 AIME, harder Prove it Belarus

Solve the equation in nonnegative integers aa, bb, cc:
3a+2b+2015=3c! 3^a + 2^b + 2015 = 3c!

Solution

(Solution by A. Sheremet.) Since 3c!>20153c! > 2015, we have c6c \ge 6.

1) For a=0a = 0 from the given equation
3a+2b+2015=3c!(1) 3^a + 2^b + 2015 = 3c! \quad (1)
we have 2b=3(c!672)2^b = 3(c! - 672), which is impossible.

2) Let a=1a = 1. If c=6c = 6, then 2b=1422^b = 142 -- there are no solutions. If c7c \ge 7, then (1) implies 2b+201872^b + 2018 \ge 7, or 2b+20(mod7)2^b + 2 \equiv 0 \pmod 7, which is impossible.

3) Let now a2a \ge 2. Then 3a93^a \ge 9, hence from (1) it follows that 2b1(mod9)2^b \equiv 1 \pmod 9. Therefore b6b \ge 6.
If b=0b = 0, then we have 3a+2016=3c!3^a + 2016 = 3c!, which is impossible for c=6c = 6; for c7c \ge 7 the obtained equality implies 3a73^a \ge 7, which is false.
Therefore b6b \ge 6 and from (1) it follows that 3a1(mod16)3^a \equiv 1 \pmod{16} which implies a4a \ge 4. Let a=4ta = 4t, b=6qb = 6q. Then (1) becomes
81t+64q+2015=3c!(2) 81^t + 64^q + 2015 = 3c! \quad (2)
The case c=6c = 6 leads to t=q=1t = q = 1, i.e., a=4a = 4, b=6b = 6. The case c7c \ge 7 leads to the congruence 4t+1q+60(mod7)4^t + 1^q + 6 \equiv 0 \pmod 7, which is impossible. Thus the only solution is (a,b,c)=(4,6,6)(a, b, c) = (4, 6, 6).

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