# Nested Summation

Using below code i am getting result shown in below image but its not properly displayed for

1. ex space on either side of =.
2. First summation sign is small.
3. Brackets ( ) are small.
4. All items are not placed properly with spaces between them.

How to improve all these things?

T(n)= $\sum\limits_{i=3}^{n-4}(\sum\limits_{j=4}^{n-1} 1 + \sum\limits_{k=10}^{n-3} 1)$

MWE

\documentclass{article}

\usepackage{fancyhdr} % Required for custom headers
\usepackage{lastpage} % Required to determine the last page for the footer
\usepackage{extramarks} % Required for headers and footers
\usepackage[usenames,dvipsnames]{color} % Required for custom colors
\usepackage{graphicx} % Required to insert images
\usepackage{listings} % Required for insertion of code
\usepackage{courier} % Required for the courier font
\usepackage{lipsum} % Used for inserting dummy 'Lorem ipsum' text into the template
\DeclarePairedDelimiter\Floor\lfloor\rfloor
\DeclarePairedDelimiter\Ceil\lceil\rceil

% Margins
\topmargin=-0.45in
\evensidemargin=0in
\oddsidemargin=0in
\textwidth=6.5in
\textheight=9.0in
T(n)= $\sum\limits_{i=3}^{n-4}(\sum\limits_{j=4}^{n-1} 1 + \sum\limits_{k=10}^{n-3} 1)$