A direct sum of GelfandÂ–NaimarkÂ–Segal representations is the same as being able to represent a C-star-algebra as bounded, linear operators on a space named for this man.
This man is the alphabetically later namesake of the quantity proportional to the integral over the whole of space-time of the Ricci scalar times the square root of the negative determinant of the metric tensor matrix. Einstein and this man name the action functional that yields the Einstein  eld equations through the principle of least action. A direct sum of GelfandÂ–NaimarkÂ–Segal representations is the same as being able to represent a C-star-algebra as bounded, linear operators on a space named for this man. He names the complex vector space, possessing an inner product, of all square-integrable functions. The state of a particle is de ned by a ket in his namesake "spaces." For 10 points, what German mathematician's "spaces" are used to formulate quantum mechanics?
This man is the alphabetically later namesake of the quantity proportional to the integral over the whole of space-time of the Ricci scalar times the square root of the negative determinant of the metric tensor matrix.
David Hilbert [accept EinsteinÂ–Hilbert action or Hilbert space(s)]