Geodesic
The numerical range of linear relations in spaces with an indefinite metric
Žurnal matematičeskoj fiziki, analiza, geometrii, Tome 7 (2000) no. 1, pp. 115-118 
Ts. Bayasgalan. The numerical range of linear relations in spaces with an indefinite metric. Žurnal matematičeskoj fiziki, analiza, geometrii, Tome 7 (2000) no. 1, pp. 115-118. http://geodesic.mathdoc.fr/item/JMAG_2000_7_1_a5/
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     title = {The numerical range of linear relations in spaces with an indefinite metric},
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Voir la notice de l'article provenant de la source Math-Net.Ru

It is shown that the numerical range of linear relations in spaces with an indefinite metric is always convex.