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| In [[mathematics]], the (linear) '''Peetre theorem,''' named after [[Jaak Peetre]], is a result of [[functional analysis]] that gives a characterisation of [[Smooth Operator|differential operator]]s in terms of their effect on generalized [[function space]]s, and without mentioning [[derivative|differentiation]] in explicit terms. The Peetre theorem is an example of a [[finite order theorem]] in which a function or a [[functor]], defined in a very general way, can in fact be shown to be a polynomial because of some extraneous condition or symmetry imposed upon it.
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| This article treats two forms of the Peetre theorem. The first is the original version which, although quite useful in its own right, is actually too general for most applications.
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| == The original Peetre theorem ==
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| Let ''M'' be a [[smooth manifold]] and let ''E'' and ''F'' be two [[vector bundles]] on ''M''. Let
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| :<math>\Gamma^\infty (E),\ \hbox{and}\ \Gamma^\infty (F)</math>
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| be the spaces of [[smooth section]]s of ''E'' and ''F''. An ''operator''
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| :<math>D:\Gamma^\infty (E)\rightarrow \Gamma^\infty(F)</math>
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| is a [[sheaf (mathematics)|morphism of sheaves]] which is linear on sections such that the [[support (mathematics)|support]] of ''D'' is [[non-increasing]]: ''supp Ds'' ⊆ ''supp s'' for every smooth section ''s'' of ''E''. The original Peetre theorem asserts that, for every point ''p'' in ''M'', there is a neighborhood ''U'' of ''p'' and an integer ''k'' (depending on ''U'') such that ''D'' is a [[differential operator]] of order ''k'' over ''U''. This means that ''D'' factors through a linear mapping ''i''<sub>''D''</sub> from the ''k''-[[jet (mathematics)|jet of sections]] of ''E'' into the space of smooth sections of ''F'':
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| :<math>D=i_D\circ j^k</math>
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| where | |
| :<math>j^k:\Gamma^\infty E\rightarrow J^kE</math>
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| is the ''k''-jet operator and
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| :<math>i_D:J^kE\rightarrow F</math>
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| is a linear mapping of vector bundles.
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| === Proof ===
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| The problem is invariant under local diffeomorphism, so it is sufficient to prove it when ''M'' is an open set in '''R'''<sup>''n''</sup> and ''E'' and ''F'' are trivial bundles. At this point, it relies primarily on two lemmas:
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| *'''Lemma 1.''' If the hypotheses of the theorem are satisfied, then for every ''x''∈''M'' and ''C'' > 0, there exists a neighborhood ''V'' of ''x'' and a positive integer ''k'' such that for any ''y''∈''V''\{''x'') and for any section ''s'' of ''E'' whose ''k''-jet vanishes at ''y'' (''j<sup>k</sup>s''(''y'')=0), we have |''Ds''(''y'')|<C.
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| *'''Lemma 2.''' The first lemma is sufficient to prove the theorem.
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| We begin with the proof of Lemma 1.
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| :Suppose the lemma is false. Then there is a sequence ''x''<sub>k</sub> tending to ''x'', and a sequence of very disjoint balls ''B''<sub>k</sub> around the ''x''<sub>''k''</sub> (meaning that the geodesic distance between any two such balls is non-zero), and sections ''s''<sub>k</sub> of ''E'' over each ''B''<sub>''k''</sub> such that ''j''<sup>''k''</sub>s<sub>k</sub>(''x''<sub>''k''</sub>)=0 but ''Ds''<sub>k</sub>(''x''<sub>''k''</sub>)≥C>0.
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| :Let ρ(''x'') is a standard [[bump function]] for the unit ball at the origin: a smooth real-valued function which is equal to 1 on ''B''<sub>1/2</sub>(0), which vanishes to infinite order on the boundary of the unit ball.
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| :Consider every other section ''s''<sub>''2k''</sub>. At ''x''<sub>''2k''</sub>, these satisfy
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| ::''j''<sup>''2k''</sub>''s''<sub>2k</sub>(''x''<sub>''2k''</sub>)=0.
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| :Suppose that ''2k'' is given. Then, since these functions are smooth and each satisfy ''j''<sup>2k</sub>(''s''<sub>2k</sub>)(''x''<sub>2k</sub>)=0, it is possible to specify a smaller ball ''B′''<sub>δ</sub>(''x''<sub>''2k''</sub>) such that the higher order derivatives obey the following estimate:
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| ::<math>\sum_{|\alpha|\le k}\ \sup_{y\in B'_\delta(x_{2k})} |\nabla^\alpha s_k(y)|\le \frac{1}{M_k}\left(\frac{\delta}{2}\right)^k</math>
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| :where
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| ::<math>M_k=\sum_{|\alpha|\le k}\sup |\nabla^\alpha\rho|.</math>
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| :Now
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| ::<math>\rho_{2k}(y):=\rho\left(\frac{y-x_{2k}}{\delta}\right)</math>
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| :is a standard bump function supported in ''B′''<sub>δ</sub>(''x''<sub>''2k''</sub>), and the derivative of the product ''s''<sub>2k</sub>ρ<sub>2k</sub> is bounded in such a way that
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| ::<math>\max_{|\alpha|\le k}\ \sup_{y\in B'_\delta(x_{2k})}|\nabla^\alpha (\rho_{2k}s_{2k})|\le 2^{-k}.</math>
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| :As a result, because the following series and all of the partial sums of its derivatives converge uniformly
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| ::<math>q(y)=\sum_{k=1}^\infty\rho_{2k}(y)s_{2k}(y),</math>
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| :''q''(''y'') is a smooth function on all of ''V''.
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| :We now observe that since ''s''<sub>2k</sub> and <math>\rho</math><sub>2k</sub>''s''<sub>2k</sub> are equal in a neighborhood of ''x''<sub>2k</sub>,
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| ::<math>\lim_{k\rightarrow\infty}|Dq(x_{2k})|\ge C</math>
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| :So by continuity |''Dq''(''x'')|≥ C>0. On the other hand,
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| ::<math>\lim_{k\rightarrow\infty}Dq(x_{2k+1})=0</math>
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| :since ''Dq''(''x''<sub>2k+1</sub>)=0 because ''q'' is identically zero in ''B''<sub>2k+1</sub> and ''D'' is support non-increasing. So ''Dq''(''x'')=0. This is a contradiction.
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| We now prove Lemma 2.
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| :First, let us dispense with the constant ''C'' from the first lemma. We show that, under the same hypotheses as Lemma 1, |Ds(y)|=0. Pick a ''y'' in ''V''\{''x''} so that ''j''<sup>k</sup>''s''(y)=0 but |''Ds''(''y'')|=''g''>0. Rescale ''s'' by a factor of 2''C''/g. Then if ''g'' is non-zero, by the linearity of ''D'', |''Ds''(''y'')|=2''C''>''C'', which is impossible by Lemma 1. This proves the theorem in the punctured neighborhood ''V''\{''x''}.
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| :Now, we must continue the differential operator to the central point ''x'' in the punctured neighborhood. ''D'' is a linear differential operator with smooth coefficients. Furthermore, it sends germs of smooth functions to germs of smooth functions at ''x'' as well. Thus the coefficients of ''D'' are also smooth at ''x''.
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| == A specialized application ==
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| Let ''M'' be a [[compact (topology)|compact]] [[smooth manifold]] (possibly with [[manifold|boundary]]), and ''E'' and ''F'' be finite dimensional [[vector bundle]]s on ''M''. Let
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| :<math>\Gamma^\infty (E)</math>
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| be the collection of [[smooth section]]s of ''E''. An ''operator''
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| :<math>D:\Gamma^\infty (E)\rightarrow F</math>
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| is a smooth function (of [[Fréchet manifold]]s) which is linear on the fibres and respects the base point on ''M'':
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| :<math>\pi\circ D_p=p.</math>
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| The Peetre theorem asserts that for each operator ''D'', there exists an integer ''k'' such that ''D'' is a [[differential operator]] of order ''k''. Specifically,
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| :<math>D=i_D\circ j^k:J^k(E)\rightarrow F</math>
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| is a mapping from the [[jet (mathematics)|jet]]s of sections of ''E'' to the bundle ''F''. See also [[differential operator#coordinate-independendent description|intrinsic differential operators]].
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| == References ==
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| * Peetre, J., [http://www.mscand.dk/article.php?id=1545 ''Une caractérisation abstraite des opérateurs différentiels''], Math. Scand. '''7''' (1959), 211-218.
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| * Peetre, J., Rectifications à l'article ''Une caractérisation abstrait des opératuers différentiels'', Math. Scand. '''8''' (1960), 116-120.
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| * [[Chuu-Lian Terng|Terng, C.L.]], ''Natural vector bundles and natural differential operators'', American J. of Math. '''100''' (1978), 775-828.
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| [[Category:Articles containing proofs]]
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| [[Category:Differential operators]]
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| [[Category:Functional analysis]]
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| [[Category:Theorems in functional analysis]]
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