Truncating the series. In section 5.2, I truncated a Taylor series abruptly; the questions below help you to justify that step.
To simplify the analysis, specify that the reservoir is a monoatomic classical ideal gas of N spinless atoms
To specify an energy eigenstate, we will need to specify N triple {n_x,n_y,n_z} in the spirit of section 4.1 There will be one quantum state per unit volume in an enlarged mathematical space of 3 N dimensions. (That statement would be literally correct if the N particles were distinguishable. The actual indistinguishability has no qualitative effect on the subsequent reasoning, and so we ignore it.) The energy E will continue to be proportional to n^2 , where is n is the “radius” vector in that space. The integral in the analog of equation (4.6), however, will contain the factor n^(3N-1) dn, for a “volume” in a space of 3 N dimensions must go as n^3N, The density of states must have the form