- I need to do this.
- I need do this.
My English grammar knowledge tells me that "need" doesn't have the same status as the modal verbs "may", "can", "should" and what not. Hence the second usage where two verbs appear consecutively is incorrect.
But yesterday, my native English speaking friends (Americans) told me that "I need compute this." is a perfectly grammatical sentence, and one is simply omitting the "to". How can this be? Is it a colloquial usage but grammatically incorrect, or is it grammatically correct? If it is grammatically correct, is it because "need" is a semi-modal verb?
EDIT: in particular, is it okay to use "need compute" in a scientific paper?
EDIT2: the exact phrase that raised the question was "The advantage of this representation is that we need only compute sums and products"