Article history: Received 23 May 2012 Received in revised form 17 September 2012 Accepted 2 November 2012 Available online xxxx

Article history: Received 25 October 2011 Received in revised form 16 March 2012 Accepted 23 June 2012 Available online 2 July 2012

Article history: Received 27 June 2012 Received in revised form 23 November 2012 Accepted 28 December 2012 Available online 3 January 2013 Communicated by G. Battle

Article history: Received 16 December 2011 Received in revised form 23 March 2012 Accepted 8 April 2012 Available online 11 April 2012 Communicated by R. Wu

Article history: Received 23 October 2011 Received in revised form 20 February 2012 Accepted 20 February 2012 Available online 9 March 2012

Article history: Received 17 December 2010 Received in revised form 23 April 2012 Accepted 26 April 2012 Available online 4 May 2012

Article history: Received 24 November 2011 Received in revised form 29 February 2012 Accepted 17 May 2012 Available online 23 May 2012