Diophantine Factorial


নিচের সমীকরণটির সকল সম্ভাব্য অঋণাত্মক পূর্ণসাংখ্যিক সমাধান $(x,y)$ নির্ণয় করোঃ 

   $$ x! + 2^y = (x+1)!$$

  দ্রষ্টব্যঃ  $x! = x\cdot (x-1)!$ and $0! = 1$. উদাহরণস্বরুপ, $5! = 5\times 4\times 3\times 2\times 1 = 120$। 


Proof Based Problems  


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Solution

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$x! + 2^y = (x+1)! $

$ \implies 2^y = (x+1)\cdot x! - x!$

$ \implies 2^y = x\cdot x!$


Now, if $x>2$, $3 \mid x\cdot x!$. But $3\nmid 2^y$. $\therefore x \leq 2$. $x=0$ doesn't work because $2^y = 0$ has no solutions.

By putting $x=1,2$, we obtain that $(x,y)=(1,0),(2,2)$ and we are done.

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