<http://www.w3.org/1999/02/22-rdf-syntax-ns#first>
<http://ichef-1.bbci.co.uk/news/ws/560/amz/worldservice/live/assets/images/2015/11/04/151104015501_matematicas_304x171_thinkstock.jpg>
<http://www.bbc.com/mundo/noticias/2015/11/151103_eeuu_problema_matematico_polemica_jg>