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The Hilbert space l2 has the orthonormal basis (en) with en(m) = dmn;m, n Î N.
Hence its Hilbert dimension is À0. But the set of all sequences xa = < 1,a,a2, a3, ¼ > , 0 < a < 1 is a linearly independent uncountable subset of l2.
Thus the Hamel dimension of l2 isn't À0.
Comment. This Hilbert dimension is probably the only one which this can happen.