On some $D(u^2)$-triples and quadruples of triangular numbers

We classify all $D(u^2)$-triples of triangular numbers whose smallest element is $T_{4u-1}$ or $T_{4u}$. We study the regularity of $D(N)$-triples of triangular numbers. We also show that infinitely many positive integers $u$ admit an all-triangular $D(u^2)$-quadruple. For every positive integer $k$