Ditchev, A. “SOME RESULTS ON THE UPPER-SEMILATTICE OF R.E. $m$-DEGREES INSIDE A SINGLE R.E. $tt$-DEGREE”. Ann. Sofia Univ. Fac. Math. And Inf., vol. 96, Dec. 2004, pp. 23-34, https://annual.uni-sofia.bg/index.php/fmi/article/view/158.