I am trying to solve the following functional equation, and could use some help.2f(x)=f(ax)
For some a∈R. By repeated adding 2f(x) together we notice that 2nf(x)=f(anx).
Also 2mf(a−mx)=f(x)⇒12mf(x)=f(a−mx).
Putting it all together I have for all m,n∈N, nmf(x)=2n2mf(x)=f(an−mx)
I am unsure of how to proceed from here.
Answer
I give only some hints for the case a>0, a≠1, 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 x∈R. 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.
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