Published online by Cambridge University Press: 20 November 2018
Inspired by a construction by Bump, Friedberg, and Ginzburg of a two-variable integral representation on   $\text{GS}{{\text{p}}_{4}}$  for the product of the standard and spin
 $\text{GS}{{\text{p}}_{4}}$  for the product of the standard and spin   $L$ -functions, we give two similar multivariate integral representations. The first is a three-variable Rankin-Selberg integral for cusp forms on
 $L$ -functions, we give two similar multivariate integral representations. The first is a three-variable Rankin-Selberg integral for cusp forms on   $\text{PG}{{\text{L}}_{4}}$  representing the product of the
 $\text{PG}{{\text{L}}_{4}}$  representing the product of the   $L$ -functions attached to the three fundamental representations of the Langlands
 $L$ -functions attached to the three fundamental representations of the Langlands   $L$ -group
 $L$ -group   $\text{S}{{\text{L}}_{\text{4}}}\left( \text{C} \right)$ . The second integral, which is closely related, is a two-variable Rankin-Selberg integral for cusp forms on
 $\text{S}{{\text{L}}_{\text{4}}}\left( \text{C} \right)$ . The second integral, which is closely related, is a two-variable Rankin-Selberg integral for cusp forms on   $\text{PGU}\left( 2,\,2 \right)$  representing the product of the degree
 $\text{PGU}\left( 2,\,2 \right)$  representing the product of the degree   $8$  standard
 $8$  standard   $L$ -function and the degree
 $L$ -function and the degree   $6$  exterior square
 $6$  exterior square   $L$ -function.
 $L$ -function.