This set is formated for the Drafts app, although I haven't tested them yet, but they should work. These strings should work in Drafts or Drafts4 since the schemes are the same.
http://www.openproblemgarden.org/search/[[selection]]
http://mathworld.wolfram.com/search/?query=[[selection]]&x=0&y=0
http://math.wikia.com/wiki/Special:Search?search=[[selection]]&fulltext=Search
https://tqft.net/mlp_wiki/index.php?search=[[selection]]&go=Go&title=Special%3ASearch
http://groupprops.subwiki.org/w/index.php?search=[[selection]]&title=Special%3ASearch&go=Go
http://www.encyclopediaofmath.org/index.php?title=Special%3ASearch&search=[[selection]]&button=
http://timescalewiki.org/index.php?search=[[selection]]&button=&title=Special%3ASearch
http://ncatlab.org/nlab/search?query=[[selection]]
http://plato.stanford.edu/search/searcher.py?query=[[selection]]
https://archive.org/search.php?query=[[selection]]
https://proofwiki.org/w/index.php?search=[[selection]]&title=Special%3ASearch&go=Go
https://scholar.google.com/scholar?hl=en&q=[[selection]]&btnG=&as_sdt=1%2C16&as_sdtp=