Foursquare is principally funded by Union Square Ventures , Andreessen Horowitz , O'Reilly AlphaTech Ventures , and Spark Capital . The company raised $ million in its Series A round and $20 million in its Series B round .   On June 24, 2011, Foursquare raised $50 million on a $600 million valuation.  Their Series D round funding of $41 million was announced on April 11, 2013 and led by Silver Lake Partners .  Unlike prior funding rounds that had been financed via equity , a good portion of Series D came in the form of convertible debt from existing investors. 
The word 'efficiently' here means up to polynomial-time reductions . This thesis was originally called Computational Complexity-Theoretic Church–Turing Thesis by Ethan Bernstein and Umesh Vazirani (1997). The Complexity-Theoretic Church–Turing Thesis, then, posits that all 'reasonable' models of computation yield the same class of problems that can be computed in polynomial time. Assuming the conjecture that probabilistic polynomial time ( BPP ) equals deterministic polynomial time ( P ), the word 'probabilistic' is optional in the Complexity-Theoretic Church–Turing Thesis. A similar thesis, called the Invariance Thesis , was introduced by Cees F. Slot and Peter van Emde Boas. It states: "Reasonable" machines can simulate each other within a polynomially bounded overhead in time and a constant-factor overhead in space .  The thesis originally appeared in a paper at STOC '84, which was the first paper to show that polynomial-time overhead and constant-space overhead could be simultaneously achieved for a simulation of a Random Access Machine on a Turing machine.