Motivated by a result of Ky Fan in 1965, we establish a characterization of a left amenable F-algebra (which includes the group algebra and the Fourier algebra of a locally compact group and quantum group algebras, or more generally the predual algebra of a Hopf von Neumann algebra) in terms of a finite dimensional invariant subspace property. This is done by first revealing a fixed point property for the semigroup of norm one positive linear functionals in the algebra. Our result answers an open question posted in Tokyo in 1993 by the first author (see [25, Problem 5]). We also show that the left amenability of an ideal in an F-algebra may determine the left amenability of the algebra. 1 2 A. T.-M. LAU AND Y. ZHANG invertible linear mappings that preserve the quadratic form J( x) = x 2 + y 2 + z 2 − c 2 t 2 , x = (x, y, z, t) ∈ R 4 where the constant c is the speed of light. Quantity J represents the space time interval. It is a well known fact that for any Lorentz transformation T there is a three dimensional subspace V of R 4 which is T -invariant and positive (in the sense that T ( x) = x and J( x) ≥ 0 for all x ∈ V ). L. S. Pontrjagin [43], I. S. Iovihdov [20], M. G. Krein [22, 21] and M. A. Naimark [40, 41] investigated infinite-dimension versions of the above invariant subspace property, and Naimark finally established the following theorem in 1963 [40].