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The notion of pushforward in mathematics is "dual" to the notion of pullback, and can mean a number of different but closely related things. Pushforward (differential), the differential of a smooth map between manifolds, and the "pushforward" operations it defines
This linear map is known as the pullback (by ), and is frequently denoted by . More generally, any covariant tensor field – in particular any differential form – on N {\displaystyle N} may be pulled back to M {\displaystyle M} using ϕ {\displaystyle \phi } .
The pullback bundle is an example that bridges the notion of a pullback as precomposition, and the notion of a pullback as a Cartesian square. In that example, the base space of a fiber bundle is pulled back, in the sense of precomposition, above. The fibers then travel along with the points in the base space at which they are anchored: the ...
Let : be a smooth map of smooth manifolds. Given , the differential of at is a linear map : from the tangent space of at to the tangent space of at (). The image of a tangent vector under is sometimes called the pushforward of by .
In general, any measurable function can be pushed forward. The push-forward then becomes a linear operator, known as the transfer operator or Frobenius–Perron operator.In finite spaces this operator typically satisfies the requirements of the Frobenius–Perron theorem, and the maximal eigenvalue of the operator corresponds to the invariant measure.
People often use the simple mnemonic spring forward, fall back to remember to set clocks forward one hour (e.g., from 2 a.m. to 3 a.m.) in the spring and backward one hour (e.g., from 2 a.m. to 1 ...
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Some pullback motors, usually intended for racing in pairs, have used a catch and release mechanism to retain their springs. These may be wound separately, then launched together by releasing their spring triggers. Darda use such a mechanism for their Stop'n'Go motor. [4] This is pre-wound, then releases automatically when shunted from behind.