Can you solve this function problem with a hint and the given condition?

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Homework Help Overview

The discussion revolves around a functional equation involving a function f defined from the real numbers to itself, with specific properties including f(1) not being zero and the equations f(x+y) = f(x) + f(y) and f(xy) = f(x)(y). The goal is to demonstrate that f(x) = x for all x in R.

Discussion Character

  • Exploratory, Conceptual clarification, Mathematical reasoning, Assumption checking

Approaches and Questions Raised

  • Participants explore the implications of f(1) not being zero, suggesting that this allows for division in equations. They discuss initial steps involving f(1 + 0) and f(0), questioning the validity of dividing by f(0) and noting that it leads to f(0) = 0. There is mention of using induction to show f(n) = n f(1) for integers and the need to extend this to all real numbers.

Discussion Status

The conversation is ongoing, with participants sharing insights and approaches without reaching a consensus. Some have provided guidance on potential steps, while others are awaiting further contributions from the original poster to advance the discussion.

Contextual Notes

There is a noted assumption regarding a possible typo in the original problem statement, where f(xy) = f(x)f(y) is assumed instead of f(xy) = yf(x). This assumption influences the direction of the discussion.

barathiviji
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give me solution to this problem...

Let f be a function from the set of real numbers R to R such that f(1) is not equal to 0 , and f(x+y)=f(x)+f(y), f(xy)=f(x)(y) for all x,y belongs to R. Then show that f(x)=x for all x in R
 
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I don't have a solution, but I'm pretty certain you need to use the information that f(1) is not zero. This suggests to me that you would be able to divide both sides of an equation by f(1).

Some things to start with are
f(1 + 0) = f(1) + f(0) <I don't know what to do with this.>
Now f(0) = f(1*0), so f(0) = f(1)*f(0) so f(0)/f(0) = f(1).
That might be a start for you.
 
Mark44 said:
I don't have a solution, but I'm pretty certain you need to use the information that f(1) is not zero. This suggests to me that you would be able to divide both sides of an equation by f(1).

Some things to start with are
f(1 + 0) = f(1) + f(0) <I don't know what to do with this.>
Now f(0) = f(1*0), so f(0) = f(1)*f(0) so f(0)/f(0) = f(1).
That might be a start for you.

Well the first equation in fact tells you f(0) = 0, so it probably wasn't a good idea to divide by that in the second line =].

It should be fairly straight forward to show by induction that f(n) = n f(1), integral n. Now if we use that fact in f(xy) = f(x)f(y), we can see that f(1) = 1, so now we can conclude f(n) = n for integer n. EDIT: Throughout this thread we have assumed the OP made a typo meaning f(xy) = f(x) f(y), though he wrote f(xy) = yf(x). If that is the actual question, then this is quite trivial.
The problem we have left is to show it for all real numbers other than just the integers.
 
Last edited:
Gib Z said:
The problem we have left is to show it for all real numbers other than just the integers.
Next on to the rationals! Alas, we should wait for Barathiviji to provide some work in this endeavor.
 
Mark44 said:
I don't have a solution, but I'm pretty certain you need to use the information that f(1) is not zero. This suggests to me that you would be able to divide both sides of an equation by f(1).

Some things to start with are
f(1 + 0) = f(1) + f(0) <I don't know what to do with this.>
Now f(0) = f(1*0), so f(0) = f(1)*f(0) so f(0)/f(0) = f(1).
That might be a start for you.

Sorry, I must have gotten in a hurry. I first said to divide by f(1), but later divided by f(0).
 

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