but which will be discussed, has a T$_{c}$ ≈ 2 . 4 mK near the melting pressure with γ of about 50 Joule/moleK $^{2}$. Dividing the latter by the ratio of the He 3 mass to the electronic mass, the equivalent electronic γ , is about 6 mJ/moleK $^{2}$. 

The observed maximum T$_{c}$ from e-ph interactions, realized in metals like Pb − Bi alloys, Nb$_{3}$Sn , MgB$_{2}$ , etc., is of order θ$_{d}$/ 10. Here θ$_{D}$ is the Debye temperature. For the e-e promoted cases, T$_{c}$ , for both the heavy fermions and the Cuprates is O ( E$_{f}$/ 10 $^{2}$) , where E$_{f}$ is the renormalized Fermi-energy. For He $^{3}$, the same ratio is about E$_{f}$/ 10 $^{3}$. We will discuss that there are good reasons for these orders of magnitudes. I will also argue that He 3 is about as high a value of T$_{c}$/E$_{f}$ as one is likely to get from fermion interactions unless one is at proximity to a quantum critical point of a special kind, which is the case for both the Cuprates and the heavy-fermion compounds. 

## B. Organization and Summary of this overview 

I will first present in Sec. II, the minimal necessary summary of the theoretical back- ground of aspects relevant to pairing symmetry and T$_{c}$ . I will start with the general vertex in the pairing channel and discuss the basis for its approximation by the form generally used. I will then present the parameters that go into determining T$_{c}$ through Eliashberg equations for general forms of effective electron-electron interaction vertices. The param- eters determining T$_{c}$ for the electron-phonon problem and in the general case will also be summarized. The favored ”angular-momentum” state ℓ (more properly the irreducible rep- resentation of the point group) of pairing will be extracted from the momentum and the spin-dependence of the interaction vertices. Also discussed here, following Kohn and Lut- tinger [8], is the instructive case of superconductivity parameters in jellium for pairing in any angular momentum. 

This, the determination of the favored channel of pairing is the easy part of the prob- lem. The hard part is the discussion of the factors determining T$_{c}$ . We will review that they are the spectral strength of the fluctuations exchanged, their characteristic energy and their distribution with respect to thermal energies, the coupling of these fluctuations to fermions and the density of states of the latter over the scale of the characteristic energy of the fluctuations. One of the important points emphasized here is that these factors are not independent. This is explicitly evident in the empirical evidence presented (and its deriva- 

