**Acknowledgements**

I would like to particularly thank Dr. Jiro Inaida. I learned the rigorous formulation of arithmetic, etc. for fuzzy numbers by level-cut sets from him. I also deeply thank my co-researcher, senior and friend, Dr. Hiroaki Uesu. My research on type-2 fuzzy numbers is largely due to him.

*Review of Type-1 and Type-2 Fuzzy Numbers DOI: http://dx.doi.org/10.5772/intechopen.110495*
