13. In the parallelepiped $A E C D-A_{1} B_{1} C_{1} D_{1}$, it is known that the diagonals $A_{1} C=4, B_{1} D=2$. If a point $P$ in space satisfies $P A_{1}=3, P C=5$, then $P B_{1}^{2}+P D^{2}=$
13. 28 .