A number theory problem by Prayas Rautray

How many real solutions are there for x, if x + 2 x + 4 x + 8 x + 16 x + 32 x = 12345 \lfloor x\rfloor+\lfloor 2x\rfloor+\lfloor 4x\rfloor+\lfloor 8x\rfloor+\lfloor 16x\rfloor+\lfloor 32x\rfloor=12345


The answer is 0.

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1 solution

Mark Hennings
Aug 14, 2017

The function f ( x ) = j = 0 5 2 j x x R f(x) \; = \; \sum_{j=0}^5 \big\lfloor 2^j x\big\rfloor \hspace{2cm} x \in \mathbb{R} is an increasing function of x x , and f ( 196 ) = 12348 f(196) = 12348 . If 196 1 32 < x < 196 196 - \tfrac{1}{32} < x < 196 then it is clear that 2 j x = 2 j × 196 1 0 j 5 \big\lfloor 2^j x \big\rfloor \; = \; 2^j \times 196 - 1 \hspace{2cm} 0 \le j \le 5 and hence that f ( x ) = f ( 196 ) 6 = 12342 f(x) \,= \, f(196) - 6 \,=\, 12342 . Thus f ( x ) 12342 f(x) \le 12342 for x < 196 x < 196 and f ( x ) 12348 f(x) \ge 12348 for x 196 x \ge 196 , so the number of solutions to the equation f ( x ) = 12345 f(x) = 12345 is 0 \boxed{0} .

Mark Hennings is brilliant

James Wilson - 3 years, 9 months ago

Absolutely BRILLIANT!!!

Prayas Rautray - 3 years, 9 months ago

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