Lets start Preparing hard for KVPY SX!(1)

Algebra Level 5

If a function f : R R f : R \rightarrow R is given by the functional equation,

f ( x f ( y ) ) = f ( f ( y ) ) + x f ( y ) + f ( x ) 1 \large{f(x-f(y))=f(f(y))+xf(y)+f(x)-1} .

Now let's take the function f ( x ) f(x) where x x is obviously a real number as per our definition of the function.

  • If the function is injective, take z = 2 z=2
  • If the function is onto, take z = 3 z=3
  • If the function is into, take z = 1 z=1 .

If the value of

0 z f 1 ( t ) . d t = a m / n b \displaystyle{\int_{0}^{|z|} f^{-1}(t). dt = \dfrac{a^{{m/n}}}{b}}

Here, a is a prime number. And gcd(a,b)=1 . ||Find the value of f ( 1 ) + f ( 2 ) + a + b + m + n + z f(1)+f(2)+a+b+m+n+|z| .

Details and Assumptions


The answer is 10.5.

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1 solution

Aaghaz Mahajan
Apr 17, 2018

Nice question @Md Zuhair
After taking f(y) = 0 , we can observe that f(0) = 1........ Next, putting y as 0 we obtain a recurrence relation......after determining some values i.e. f(1) = 1/2 and f(-1) = 1/2....... We find out that the function is
f(x) = 1 - (x^2)/2...........
The rest is then easy......!!


Thanks bro.

Md Zuhair - 3 years, 1 month ago

But there is one thing. While making the question I had to think more than solvers .... As if I would have kept z=2 for into... Then the integral would have been undefined as it's not invertible till that

Md Zuhair - 3 years, 1 month ago

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Haan voh bhi hai.....but wait a sec..... original question hai kya??!!

Aaghaz Mahajan - 3 years, 1 month ago

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Toh aur Kya. Koi book mein aisa qs Thori na rahta Hain.

Md Zuhair - 3 years, 1 month ago

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@Md Zuhair Sahi hai broooo!! Functional equation to main solve hi karta hun....kuch Naya idea nahi aata ....😂lol

Aaghaz Mahajan - 3 years, 1 month ago

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@Aaghaz Mahajan Hehe.......

Md Zuhair - 3 years, 1 month ago

very nyc problem @Md Zuhair

rakshith lokesh - 3 years, 1 month ago

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Thanks Bro!

Md Zuhair - 3 years, 1 month ago

How can we take f(y) = 0 without proving the function surjective??? There is clearly a flaw in the argument .......or am i missing something?

Rayyan Shahid - 3 years, 1 month ago

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U r correct. Let me see....

Md Zuhair - 3 years, 1 month ago

@rayyan shahid Oh Man!! Yes you are right......I don't know why I did not think of it......sorry

Aaghaz Mahajan - 3 years, 1 month ago

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But I have seen this in one of the IMO problems. I guess... I have missed certain statements!

Md Zuhair - 3 years, 1 month ago

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@Md Zuhair @Md Zuhair Bro.....statements mili?? How to solve it now?? I guess Rayyan Shahid is right.......surjective waala constraint missing hai ismein......

Aaghaz Mahajan - 3 years ago

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@Aaghaz Mahajan Nahi Bhai. Khoja ni. Milte hi bol dunga

Md Zuhair - 3 years ago

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