Sunday, 26 October 2014

Solve the functional equation 2f(x)=f(ax) for some a.




I am trying to solve the following functional equation, and could use some help.2f(x)=f(ax)



For some aR. By repeated adding 2f(x) together we notice that 2nf(x)=f(anx).



Also 2mf(amx)=f(x)12mf(x)=f(amx).



Putting it all together I have for all m,nN, nmf(x)=2n2mf(x)=f(anmx)



I am unsure of how to proceed from here.



Answer



I give only some hints for the case a>0, a1, and f defined on I=]0,+[.



Put g(x)=exp(xlog2)f(exp(xloga)). Then show that f(au)=2f(u) for all u>0 is equivalent to g(x+1)=g(x) for all xR. Hence we get that the solutions are the functions f of the form f(x)=g(logxloga)exp(xlog2loga) for any g, periodic of period 1 on R.


No comments:

Post a Comment

real analysis - How to find limhrightarrow0fracsin(ha)h

How to find lim without lhopital rule? I know when I use lhopital I easy get $$ \lim_{h\rightarrow 0}...