Article history: Received 20 November 2013 Received in revised form 24 January 2014 Accepted 29 January 2014 Available online 25 February 2014

Article history: Received 29 November 2012 Received in revised form 27 February 2014 Accepted 2 March 2014 Available online 12 March 2014

Article history: Received 1 November 2013 Received in revised form 25 January 2014 Accepted 12 February 2014 Available online 15 March 2014

Article history: Received 17 April 2013 Received in revised form 30 August 2014 Accepted 26 October 2014 Available online 4 November 2014

Article history: Received 5 November 2012 Received in revised form 8 June 2014 Accepted 9 June 2014 Available online 24 June 2014

Article history: Received 17 November 2013 Received in revised form 17 October 2014 Accepted 21 October 2014 Available online 24 October 2014

Article history: Received 7 November 2013 Received in revised form 17 April 2014 Accepted 2 August 2014 Available online 7 August 2014

Article history: Received 25 October 2014 Received in revised form 26 November 2014 Accepted 29 December 2014 Available online 15 January 2015

Article history: Received 2 April 2014 Received in revised form 24 November 2014 Accepted 26 December 2014 Available online 5 January 2015

Article history: Received 4 April 2014 Accepted 20 November 2014 Available online 2 December 2014 Submitted by Y. Wei MSC: primary 15A18 secondary 15A69