Template:PlanetMath reference

WARNING : this template does not work anymore (all links redirected to title page instead of finding the intended article) due to chages at the "planetmath.org" site during year 2018. This template survived a merge and deletion proposal earlier during year 2016: Ingenpedia:Templates_for_discussion/Log/2016_September_20. A similar Template:PlanetMath still works but see the old discussion about objections.