The phrase "a number of efficient" is not correct as it is incomplete and lacks a noun to modify.
You can use it when referring to multiple efficient items or methods, but it needs to be followed by a noun.
Example: "A number of efficient solutions were proposed during the meeting."
Alternatives: "several effective" or "many efficient options".
A number of efficient numerical methods have been applied for achieving massive parallelization of all the calculation steps.
A number of efficient catalytic systems have been recently developed to convert biomass into organic acids.
Biological materials are the result of years of evolution and possess a number of efficient features and structures.
Animals have evolved a number of efficient antiviral defence mechanisms, which can serve as a source of inspiration for novel therapies.
A number of efficient reduction methods are derived in order to decrease the problem size and reduce the number of variables and constraints.
To solve the proposed possibilistic optimization model, an interactive fuzzy solution approach is developed by combining a number of efficient solution approaches from the recent literature.
However, with more data comes a need for more efficient algorithms, and the last decade has seen the publication of a number of more efficient sophisticated phylogenetic analysis methods (e.g., Guindon and Gascuel 2003; Stamatakis 2006; Zwickl 2006; Lartillot et al. 2009; de Koning et al. 2010).
Im sure there are a number of cost efficient ways to reclaim the plastic, and turn it [reused plastic] into anything you want.
Due to the importance of the problem, a number of energy efficient MAC protocols have been developed for WSNs.
When only rate-limited channel feedback is available, we design a number of computationally efficient sub-optimal power allocation algorithms to solve the associated non-convex optimization problem.
Using apps to supplement traditional taxi dispatching would create a number of more efficient options for drivers and city regulators.
