I am writing a scientific paper.

I am not sure about the usage of "assume."

Should I say "Let us assume x is real," or should I say "Let us assume x be real"?

  • 3
    Your final alternative suggests you may be confusing the construction Let's assume [statement] [is a true statement] with the textbook standard Let [variable] be [value]. – FumbleFingers Reinstate Monica Mar 22 '17 at 13:56
  • 2
    Just so it's said, "sth" is not a word. I wish its use would die. – cHao Mar 22 '17 at 18:18
  • What is "sth"...it shows up a ton in the Related sidebar – Nick T Mar 22 '17 at 20:48
  • @NickT It's an abbreviation for "something". – Catija Mar 22 '17 at 21:28

Let us assume x is real

This sounds about right.

Let us assume x be real

This is grammatically incorrect.

Let us assume x to be real

This is grammatically correct, but sounds awkward, though with more context, it could be more correct than is.

  • 4
    In some contexts, "Let x be [any] real" works also. (And FWIW, in a paper/proof, the "to be" construction above would sound perfectly fine to me.) – A C Mar 22 '17 at 17:31

Your question has made me think about this for the first time: there seems to be a distinction between verbs that merely process information about a fixed situation, and verbs that declare a position or approach to a potentially changeable situation.

For example you would say

I believe [that] X is real.

and not "I believe X be real". By contrast you should really say

I demand [that] X be real.

and not "I demand X is real".

One result of this is that the following two statements have different meaning:

I insist that X is real

means "I emphatically believe that the proposition 'X is real' describes the current situation correctly, independent of my intervention or yours", whereas

I insist that X be real

means something along the lines of "I am laying down the law, that you must ensure X is real", or more generally "X may be real or not, but if it is not real then I will refuse to proceed/cooperate with whatever we're talking about".

Your example, of assuming whether something is real, is processing information about a fixed proposition. Therefore you would not use the subjunctive. You would say "assume that X is real", not "assume that X be real".

In a mathematical paper where are you are describing your initial conditions, you can choose whether to use the metaphor of actively creating X as you define it ("let X be a real number") or the metaphor of merely describing the state of an already-defined entity ("assume X is a real number"). The former is probably more common.

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.