Using the one-dimensional potential well with realistic parameters for atomic nuclei, we illustrate the movement of the poles of the S-matrix and the transmission coefficient when the well supports an anti-bound state. We calculate the phase shift of the atomic nuclei 5He using the three-dimensional potential well and compare it with the experimental one. The paper gives an introduction to some of the properties found in realistic loosely bound and resonant nuclear systems, using mathematics accessible to undergraduate students.

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