Abstract <p>The formation of superconducting pair correlations between like nucleons in the ground state of spherical even–even nuclei is considered within a special Bogoliubov transformation. The influence of the monopole pairing interaction on the energy of single-particle states is taken into account. It is shown that the appearance of pair correlations depends on the particle number and shell structure. In open subshell nuclei the correlations exist for arbitrarily weak attractive monopole interaction and nucleon pairs are distributed over all subshells involved in the pairing interaction. It is confirmed that the superconducting pair correlations arise if the coupling constant exceeds a certain threshold value. Rough upper and lower estimates are obtained for the threshold value.</p>