Let f:Z→Z be a function satisfying the property that for any integers a, b, and c such that a+b+c=0, the equation
f(a)+f(b)+f(c)=a2+b2+c2
holds. We need to determine all such functions f.
### Step 1: Understand the Condition
Since a+b+c=0, we have c=−(a+b). Substituting c=−(a+b) into the given equation:
f(a)+f(b)+f(−(a+b))=a2+b2+(−(a+b))2.
Simplifying the right-hand side:
=a2+b2+(a+b)2=a2+b2+a2+2ab+b2=2a2+2b2+2ab.
Thus, the equation becomes:
f(a)+f(b)+f(−(a+b))=2a2+2b2+2ab.
### Step 2: Exploratory Substitution
Consider a=x, b=0, and c=−x. Thus, we have:
f(x)+f(0)+f(−x)=x2+02+(−x)2=2x2.
### Step 3: Deriving the Form of f
By the above, we have:
f(x)+f(−x)=2x2−f(0).
Now, observe the function:
We substitute f(x)=x2+cx into the condition to verify it satisfies the equation:
f(x)+f(y)+f(−(x+y))=(x2+cx)+(y2+cy)+(−(x+y))2−c(x+y).
Simplifying:
=x2+cx+y2+cy+(x+y)2+c(x+y).
=x2+y2+x2+2xy+y2+cx+cy−cx−cy.
=2x2+2y2+2xy.
Thus, the form f(x)=x2+cx satisfies the condition.
### Conclusion
The solution to the functional equation is a function of the form:
f(x)=x2+cxfor any integer c.
Thus, all functions f:Z→Z satisfying the given property are:
f(x)=x2+cx for any integer c.