Question 1209937
<pre>

Ikleyn is right that the equation is not valid for all real y, as is stated.
It is only valid for all real x and  {{{y= "" +- 1}}}.  

Note that f(1) = 0

{{{f(1*1) + 1 = 1*f(1) + f(1) + 1}}}
{{{f(1) = 2*f(1)}}}
{{{0=f(1)}}}

Edwin</pre>