"necessary assumption" is correct and is widely used in written English.
It typically refers to the idea that something must be assumed in order to be able to proceed with understanding an idea. For example, "In order to understand the theory, it was a necessary assumption that the gravitational force was constant."
The Psychological Unconscious: A Necessary Assumption for All Psychological Theory?
A necessary assumption of the SSIM is that stress and strength are independent.
A major restriction, however, was the necessary assumption of mutually incoherent sources.
Typically, inapproximability results are conditional on P!=NP, a necessary assumption since "all" combinatorial optimization problems are tractable if P=NP.
Nevertheless, it is not a necessary assumption of evolutionary theory that every feature of every organism is adapted to some purpose, much less optimally adapted.
Stock assessment scientists and fishery managers operate under the necessary assumption that the identities of species and quantities of catch from industry landing reports are known without error.
Several solid approaches to identifying mental architecture are introduced with a discussion of the necessary assumptions for these tests.
We determine that if there are pivotal countries within an international organization, necessary assumptions made for such a mechanism to work cannot be all satisfied.
As long as the necessary assumptions are satisfied, the presented analytical method can effectively be used to predict the minimum cost cell size for cylindrical cells in crossflow.
The specificities of stadium semi-outdoor spaces are summarised and the necessary assumptions made to apply the computation procedure are described.
Necessary assumptions are made to reduce the order of the strain field of the full model to a geometrically reduced order model.
