1. Dept. of Mathematics, Iowa State University, Ames, Ia., U.S.A.
2. Dept. of Mathematics and Computer Science, Grand View University, Des Moines, Iowa, U.S.A.
3. Dept. of Mathematics, Iowa State University, Ames, Ia., U.S.A.; and American Institute of Mathematics, San Jose, California, U.S.A.
4. Dept. of Mathematics, Bethel College, North Newton, Kansas, U.S.A.
5. Depts. of Mathematics and Computer Science, Iowa State University, Ames, Ia., U.S.A.
6. Dept. of Mathematics and Statistics, University of Wisconsin, La Crosse, Wisc., U.S.A.