Non-power bases

One can consider representing numbers by Sum[a[n] f[n], {n, 0,∞}] where the f[n] need not be k^{n}. So long as f[n] grows less rapidly than 2^{n} (as when f=Fibonacci or f=Prime), digits 0 and 1 will suffice, though the representation is not generally unique. (See page 1070.)