Timelike and Spacelike Ricci Collineation Vectors in String Fluid
  We study the consequences of the existence of timelike and spacelike Ricci collineation vectors (RCVs) for string fluid in the context of general relativity. Necessary and sufficient conditions are derived for a space-time with string fluid to admit a timelike RCV, parallel to $u^a$, and a spacelike RCV, parallel to $n^a$. In these cases, some results obtained are discussed. 
H. Baysal and I. Yilmaz