Que esta definiçãoé equivalenteàs duas aqui apresentadas pode ser visto no teorema 8.3.9 e 8.3.10 de [5] e na proposição 6.6.2 de [6] 10 Estamos seguindo aqui a analise feita no capítulo 2 de [14], sobre operadores diferenciais elípticos e expansão em autofunções.
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