# Image of morphism of quasi-categories

I have two questions about images of morphisms of quasi-categories.

Suppose that $$f\colon X \to Y$$ is a morphism of quasi-categories.​

1. ​Suppose that we calculate the image of $$f$$ in the category $$\mathsf{QCat}$$, using the universal property of image. Will this be a Joyal-fibrant replacement of the image as calculated in $$\mathsf{SSet}$$?
2. If $$X = N(C)$$ and $$Y = N(D)$$ are (nerves of) categories, will the image of $$f$$ be the nerve of the essential image of the functor $$C \to D$$?

In case the answer to one of these questions is no, can the statements be adjusted to become true?

• (I'm going to assume that with $\mathrm{QCat}$ you mean the 1-category of quasicategories) The answer to question 2 is no, for the same reason for which the image of a functor of categories is not the essential image: you are missing the objects that are equivalent to objects in the image but not in the image. I don't understand why you are taking the image in $\mathrm{QCat}$ though, it's not typically a useful thing to do. Maybe you can explain a bit your motivation? – Denis Nardin Nov 27 '18 at 10:36
• Does a 1-categorical image in $QCat$ even exist? – Mike Shulman Nov 27 '18 at 18:32
• I guess if you identify two $2$-simplices along their boundary (and fibrant replace), then there is no least sub-quasicategory containing the 1-skeleton, since the image of each $2$-simplex gives a sub-quasicategory. There is in fact no sub-quasicategory Joyal equivalent to the image, since every triangle commutes. – Kevin Arlin Nov 27 '18 at 19:54