Dear Ulrich,
I have the impression that your answer confuses "rigor" with
"tightness", but English is not my mother tong and I may be the
confused one.
According to my understanding of the English words, traditional
interval arithmetic is perfectly "rigorous", in the sense that it will
not lead you to wrong conclusions.
It may not be "tight", in the sense that it may
lead you to no conclusions in situations in which you
could get answers by other methods.
Which words would a native speaker use to express these
two, distinct, concepts?
regards,
walter.