On the $K$-functionals of absolutely Calderón elements of the Banach pair $( l_{1} , c_{0} )$

We characterize the absolutely Calderón elements of the canonical pair $(l_1, c_0)$ of sequence spaces in terms of the Peetre $K$-functional. This result has been known to the first author since rather long ago but the proof appears here. Also, we formulate a few unsolved problems.