Buttazzo, Giaquinta & Hildebrandt 1998, Theorem 5.17 Buttazzo, Giuseppe; Giaquinta, Mariano; Hildebrandt, Stefan (1998), One-dimensional variational problems, Oxford Lecture Series in Mathematics and its Applications, vol. 15, The Clarendon Press Oxford University Press, ISBN978-0-19-850465-8, MR1694383.
For the case of finite index sets, see, for instance, Halmos 1957, §5. For infinite index sets, see Weidmann 1980, Theorem 3.6. Halmos, Paul (1957), Introduction to Hilbert Space and the Theory of Spectral Multiplicity, Chelsea Pub. Co Weidmann, Joachim (1980), Linear operators in Hilbert spaces, Graduate Texts in Mathematics, vol. 68, Berlin, New York: Springer-Verlag, ISBN978-0-387-90427-6, MR0566954.
Berthier, M. (2020), "Geometry of color perception. Part 2: perceived colors from real quantum states and Hering's rebit", The Journal of Mathematical Neuroscience, 10 (1): 14, doi:10.1186/s13408-020-00092-x, PMC7481323, PMID32902776
However, some sources call finite-dimensional spaces with these properties pre-Hilbert spaces, reserving the term "Hilbert space" for infinite-dimensional spaces; see, e.g., Levitan 2001. Levitan, B.M. (2001) [1994], "Hilbert space", Encyclopedia of Mathematics, EMS Press.
Levitan 2001. Many authors, such as Dunford & Schwartz (1958, §IV.4), refer to this just as the dimension. Unless the Hilbert space is finite dimensional, this is not the same thing as its dimension as a linear space (the cardinality of a Hamel basis). Levitan, B.M. (2001) [1994], "Hilbert space", Encyclopedia of Mathematics, EMS Press. Dunford, N.; Schwartz, J.T. (1958), Linear operators, Parts I and II, Wiley-Interscience.
Berthier, M. (2020), "Geometry of color perception. Part 2: perceived colors from real quantum states and Hering's rebit", The Journal of Mathematical Neuroscience, 10 (1): 14, doi:10.1186/s13408-020-00092-x, PMC7481323, PMID32902776
Berthier, M. (2020), "Geometry of color perception. Part 2: perceived colors from real quantum states and Hering's rebit", The Journal of Mathematical Neuroscience, 10 (1): 14, doi:10.1186/s13408-020-00092-x, PMC7481323, PMID32902776