Since I am not so fluent with these tools and procedures I've sent to your mail address: email@example.com pdfs of two pages (from the book I'm translating):
1) Page 87_brackets bat.pdf : original coding with
2) Page 87_brackets OK.pdf: where I've replaced
\alpha_u by \beta_u
As I say on the mentioned mail, I have to keep on my work of translation, therefore I feel better from now on to use
\bigl ( and \bigr )
Again, I thank you so much for your invaluable support and I look fordward to account for your help.