Blowing up a point in projective n-space $\mathbb{P}^n$ I have clearly understood the blowing up of $\mathbb{A}^n$ at the origin and it is the zero locus of the polynomials $x_{i}y_{j} = x_{j}y_{i}$  in the mixed product space $\mathbb{A}^n \times \mathbb{P}^{n-1}$ where $(x_1,...x_n) \in \mathbb{A}^n$ and $(y_0,...,y_{n-1})\in \mathbb{P}^{n-1}$. Please help me to understand the blowing up $\mathbb{P}^n$ at a point, say $p$.
Let me try: The blow up of a point should be a closed subset of the product space $\mathbb{P}^n \times \mathbb{P}^{n-1}$. If we blowup $\mathbb{P}^n$ in two points, then the blowup will be a closed subset of the product space $\mathbb{P}^n \times \mathbb{P}^{n-1} \times  \mathbb{P}^{n-1}$. I don't know whether it is correct or not.
Is it difficult to construct the blowup of $\mathbb{P}^n$ at a point explicitly when $n > 2$?
 A: First point of view
Consider that $\mathbb{A}^n\subset \mathbb P^n$ by identifying $(x_1,\ldots ,x_n)$ with $[1:x_1.\ldots:x_n]$ .
  Then glue together the blow-up $B_0 \subset \mathbb{A}^n\times \mathbb P^{n-1}$ of $\mathbb{A}^n$ at $(0,\ldots,0)=[1:0:\ldots:0]$ and the variety $\mathbb P^n\setminus [1:0:\ldots:0] $ by identifying $((x_1,\ldots ,x_n),[x_1:\ldots:x_n])\in B_0$ with $[1:x_1:\ldots:x_n]$ whenever $(x_1,\ldots ,x_n)\neq (0,\ldots ,0)$.
The  variety $B$ obtained by this gluing process  is the required blow-up of  $\mathbb P^n$ at $[1:0:\ldots:0]$
Second  point of view
Directly describe the blow-up of  $\mathbb P^n$ at $[1:0:\ldots:0]$ as the subvariety  $B\subset \mathbb P^n \times\mathbb P^{n-1}$ defined by demanding for a pair $([x_0:x_1:\ldots:x_n],[y_1:\ldots:y_n])\in \mathbb P^n \times\mathbb P^{n-1}$ that the following  bihomogeneous conditions of bidegree $(1,1)$ hold:
$$    x_iy_j-x_jy_i   =0 \quad i,j=1,\ldots, n              $$  
(Be sure to notice that these conditions do not involve $x_0$)
