we believe it necessary

[bart-leby]

Thirdly, while reiterating our commitment to continue and intensify the work of the Triateral group, we believe it necessary to fix clearly its mandate…



Can you please explain to me why there is not used in the emphasized part of the sentence bellow is (we believe it is necessary to fix…)?

 

[Rover_KE]

Three alternatives are all acceptable here and mean the same:

'...we believe it necessary to fix...'

'...we believe it is necessary to fix...'

'...we believe it to be necessary to fix...'