REST API : is there a way to get related work items

Is there a way to get the urn of the workitems linked to a workitem ?

For example, I have a custom workitem "Function" which is linked to several standard RQM "Requirements" workitems. Using the REST API I can query all the workitems, then I can filter only the workitems of <type> "Function" and get the information related to each function.

I would like to know all the workitems of type "Requirements" which are linked to a function, but there is no tag with the related work items.