This set is formated for the 1Writer app and I tested them all so they run. They are also posted in the 1writerapp actions directory so you can install them from the actions menu.
These strings work by searching with the selected text in Safari. No need to copy just select and it should go find it.
Some of these aren't math links they are just generally useful.
http://www.openproblemgarden.org/search/[text]
http://mathworld.wolfram.com/search/?query=[text]&x=0&y=0
http://math.wikia.com/wiki/Special:Search?search=[text]&fulltext=Search
https://tqft.net/mlp_wiki/index.php?search=[text]&go=Go&title=Special%3ASearch
http://groupprops.subwiki.org/w/index.php?search=[text]&title=Special%3ASearch&go=Go
http://www.encyclopediaofmath.org/index.php?title=Special%3ASearch&search=[text]&button=
http://timescalewiki.org/index.php?search=[text]&button=&title=Special%3ASearch
http://ncatlab.org/nlab/search?query=[text]
http://plato.stanford.edu/search/searcher.py?query=[text]
https://archive.org/search.php?query=[text]
https://proofwiki.org/w/index.php?search=[text]&title=Special%3ASearch&go=Go
https://scholar.google.com/scholar?hl=en&q=[text]&btnG=&as_sdt=1%2C16&as_sdtp=