Normal Factorization in $SL(2,Z)$ and the Confluence of Singular Fibers   in Elliptic Fibrations
  In this article we obtain a result about the uniqueness of factorization in terms of conjugates of the matrix $U=(\xymatrix{1 & 1 0 & 1})$, of some matrices representing the conjugacy classes of those elements of $SL(2,Z)$ arising as the monodromy around a singular fiber in an elliptic fibration (i.e. those matrices that appear in Kodaira's list). 
Juan D. Velez, Carlos A. Cadavid