  We describe the formal analogies in the description of the inclusive
production in hard processes of hadron pairs (based on dihadron fragmentation
functions) and of a single hadron inside a jet (based on hadron-in-jet
fragmentation functions). Since several observables involving dihadron
fragmentation functions have been proposed in the past, we are able to suggest
new interesting observables involving hadron-in-jet fragmentation functions, in
lepton-hadron deep-inelastic scattering and hadronic collisions.
