Integral operators and complete families of solutions for $$\Delta _{p + 2}^2 u\left( {\underset{\raise0.3em\hbox{$\smash{\scriptscriptstyle\thicksim}$}}{x} } \right) + A\left( {r^2 } \right)\Delta _{p + 2} u\left( {\underset{\raise0.3em\hbox{$\smash{\scriptscriptstyle\thicksim}$}}{x} } \right) + B(r^2 )u\left( {\underset{\raise0.3em\hbox{$\smash{\scriptscriptstyle\thicksim}$}}{x} } \right) = 0$$