# Hahn-Banach theorem

Let be a linear manifold in a real or complex vector space . Suppose is a semi-norm on and suppose is a linear functional defined on which satisfies

(*) |

for every . Then can be extended to a linear functional on all of such that

for all . Such is an extension is not uniquely determined.

In the case of a real space the semi-norm can be replaced by a positively-homogeneous subadditive function, and the inequality (*) by the one-sided inequality , which remains valid for the extended functional. If is a Banach space, then for one can take , and then . The theorem was proved by H. Hahn (1927), and independently by S. Banach (1929).

#### References

[1] | H. Hahn, "Ueber lineare Gleichungsysteme in linearen Räume" J. Reine Angew. Math. , 157 (1927) pp. 214–229 |

[2a] | S. Banach, "Sur les fonctionelles linéaires" Studia Math. , 1 (1929) pp. 211–216 |

[2b] | S. Banach, "Sur les fonctionelles linéaires II" Studia Math. , 1 (1929) pp. 223–239 |

[3] | A.N. Kolmogorov, S.V. Fomin, "Elements of the theory of functions and functional analysis" , 1–2 , Graylock (1957–1961) (Translated from Russian) |

[4] | L.V. Kantorovich, G.P. Akilov, "Functional analysis" , Pergamon (1982) (Translated from Russian) |

#### Comments

A real-valued function is called subadditive if for all in its domain such that lies in its domain.

#### References

[a1] | N. Dunford, J.T. Schwartz, "Linear operators. General theory" , 1 , Interscience (1958) |

[a2] | G. Köthe, "Topological vector spaces" , 1 , Springer (1969) |

**How to Cite This Entry:**

Hahn–Banach theorem.

*Encyclopedia of Mathematics.*URL: http://www.encyclopediaofmath.org/index.php?title=Hahn%E2%80%93Banach_theorem&oldid=22537