Brenda S. Baker. Approximation algorithms for NP-complete problems on planar graphs. Alan Frieze and Ravi Kannan. The regularity lemma and approximation schemes for dense problems. Oscar H. Ibarra and Chul E. Kim.
Fast approximation algorithms for the knapsack and sum of subset problems.
Narendra Karmarkar and Richard M. Karp. Sanjeev Khanna and Rajeev Motwani. Tom Leighton and Satish Rao. Michael Luby and Noam Nisan. A parallel approximation algorithm for positive linear programming.