Prove the recurrence relation

hl$1,$ l$2$; m$11,$ m$2+1|$l$,$ li $=$

s

l$2($l$2+1)$ m$2($m$2+1)$

l$1($l$1+1)$ m$1($m$11)$hl$1,$ l$2$; m$1,$ m$2|$l$,$ li