In general relativity, a vanishing stress-energy tensor implies, through Einstein field equations, the vanishing of all the components of the Ricci tensor. Vacuum does not mean that the curvature of space-time is necessarily flat: the gravitational field can still produce curvature in a vacuum in the form of tidal forces and gravitational waves (technically, these phenomena are the components of the Weyl tensor). The black hole (with zero electric charge) is an elegant example of a region completely "filled" with vacuum, but still showing a strong curvature.
what produces curvature in a vacuum?
tidal forces and gravitational waves

Nintendo's near monopoly on the home video game market left it with a degree of influence over the industry. Unlike Atari, which never actively courted third-party developers (and even went to court in an attempt to force Activision to cease production of Atari 2600 games), Nintendo had anticipated and encouraged the involvement of third-party software developers; strictly, however, on Nintendo's terms. Some of the Nintendo platform-control measures were adopted by later console manufacturers such as Sega, Sony, and Microsoft, although not as stringent.
Nintendo actively encouraged  the involvement of what?
third-party software developers

The upgrade addressed a number of criticisms faced by Windows 8 upon its release, with additional customization options for the Start screen, the restoration of a visible Start button on the desktop, the ability to snap up to four apps on a single display, and the ability to boot to the desktop instead of the Start screen. Windows 8's stock apps were also updated, a new Bing-based unified search system was added, SkyDrive was given deeper integration with the operating system, and a number of new stock apps, along with a tutorial, were added. Windows 8.1 also added support for 3D printing, Miracast media streaming, NFC printing, and Wi-Fi Direct.
How many apps did the WIndows 8.1 update allow to be snapped onto a signular display?
up to four