[1] S. K. Berberian, Introduction to Hilbert Spaces, Chelsea
Publishing Company, New York, 1976.
[2] P. R. Halmos, A Hilbert Space Book, Springer-Verlag, New York,
1982.
[3] A. A. S. Jibril, On 2-normal operators, Dirasat, Natural and
Engineering Sciences 23(2) (1996), 190-195.
[4] A. A. S. Jibril, On subprojection operators, International
Mathematical Journal 4(3) (2003), 229-238.
[5] A. A. S. Jibril, On almost subprojection operators, International
Mathematical Forum 2(52) (2007), 2571-2578.
[6] A. A. S. Jibril, Further results on 2-normal operators, Mutah
Lil-Buhuth Wad-Dirasat 22(1) (2007).
[7] A. A. S. Jibril, On almost 2-normal operators, J. Math. Sci. 19(2)
(2008), 231-238.
[8] A. A. S. Jibril, On n-power normal operators, The Arabian Journal
for Science and Engineering 33(2A) (2008), 247-251.
[9] E. Kamei, Operators with skew commutative parts, Math. Japonica
25(4) (1980), 431-432.
[10] M. M. Kutkut, On the class of parahyponormal operators, J. Math.
Sci. 4(2) (1993), 73-88.