2[1]Joseph R. Zumalt, Robert W. Pasicznyuk. "The Internet and Reference Service: A real-world Test of Internet Utility", Reference & user services Quarterly 38,no .2(1998), 165 - 172.
3[2]Joseph Janes, David Carter. "Digital reference services in academic libraries", Reference & user services Quarterly 39, no.2(1999)
4[3]http:∥imagine.gsfc.nasa.gov/ask astro/ask an astronomer. html
5[4]http:∥forum.warthmore.edu./dr.math
6[5]Donald G.Frank, Katharine L.Caihoun. "The changing nature of reference and information services", Reference & user services Quarterly 15, no. 1 (2000)
724/7 Reference Project.(http:www.247ref.org/)
8Real-Time Reference for Undergraduates in Science.Mathematics,and Engineering.(http://www.library.cmu.edu/Libraries/24x7.pdf).
9Ask A Lbrarian.(http://www.earl.org.uk/ask/)
10Collaborative Digital Reference Service.(http://lcweb.loc.gov/rr/digiref/)
