Cute triplets

For x , y , z x,y,z to be prime numbers, what is the total number of ordered triplets ( x , y , z ) (x,y,z) which satisfy equation x y + 1 = z x^y +1=z ?

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4 1 infinitely many 0

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3 solutions

Adarsh Kumar
Oct 11, 2014

We know that x y + 1 y = z = x^{y}+1^{y}=z= prime.Now,as long as y = o d d y=odd x y + 1 y x^{y}+1^{y} which is a prime is divisible by x + 1 x+1 which is even (excluding 2 2 ).Thus, y = y= even.Now,the only possible case is when y = 2. y=2. x 2 + 1 = z . \Longrightarrow x^{2}+1=z. But as long as x = x= odd and z = z= odd,this can't happen.Thus,the only possible case is when x = 2. x=2. Putting x = 2 x=2 in the first equation gives z = 5. z=5. Thus,the only possible triplet x , y , z = 2 , 2 , 5. x,y,z=2,2,5.

@Sandeep Bhardwaj how about this?

Adarsh Kumar - 6 years, 8 months ago

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Its also cool. :)

Sandeep Bhardwaj - 6 years, 8 months ago

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thanx alot!!

Adarsh Kumar - 6 years, 8 months ago
Sandeep Bhardwaj
Oct 11, 2014

The only ordered pair is ( 2 , 2 , 5 ) (2,2,5)

Hint :

x,y, and z are prime numbers and prime numbers can be odd numbers only (except 2)

( o d d ) o d d = o d d . \large (odd)^{odd}=odd .

o d d + 1 = e v e n = z ( n o t p o s s i b l e ) \large odd+1=even=z (not possible) except 2

!!!

Sir,why not more (1,y,2)

y can take any prime value

Ayush Verma - 6 years, 8 months ago

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Because it is given that x,y,z are all prime numbers. You are taking x=1, but 1 is not a prime number. got it ?

Sandeep Bhardwaj - 6 years, 8 months ago

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Yes,thanks

Ayush Verma - 6 years, 7 months ago

x,y, and z are prime numbers and prime numbers can be odd numbers only (except 2) answer is (2,2,5)

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