Palantir Technologies, a Silicon Valley data start-up, said on Monday that it had filed to go public, setting up one of the largest public listings of a technology start-up since Uber made its debut last year. Palantir is one of the tech industry’s most valuable private companies, with a valuation of $20 billion... Despite persistent speculation about its prospects as a public company, Palantir had avoided listing its shares, in part because of the secretive nature of its business. A public listing would reveal a fuller picture of Palantir’s work, particularly with government agencies, for the first time.