CS 721: Introduction to Complexity Theory

Semester: Autumn 2026–27  ·  IIT Bombay
Instructor: Vishwas Bhargava
Time: 3:30 – 5 pm
Location: LT003
Complexity class diagram showing relationships between AC⁰, L, NC, P, NP, coNP, BPP, PH, PSPACE, EXP and others
Known relationships between complexity classes
About Topics Grading Assignments Lectures References

About

This course is a hike through the complexity zoo, with detours into different landscapes — space complexity, circuits, randomness, interactive proofs — and a look at how they connect. We begin with Turing machines and the classes P and NP, develop reductions and completeness, and work outward from there.

The main objective is a complexity-theoretic view of computation: how to formalize it, how to compare problems, and why establishing limits has proved so stubbornly hard.

Prerequisites A solid undergraduate background in algorithms, discrete mathematics and general mathematical maturity. Some familiarity with the theory of computation (Turing machines, P and NP) will be helpful but not required.
Teaching Assistants Aditya Neeraje (23b0940 [at] iitb [dot] ac [dot] in)
Pavankumar N (25m0803 [at] iitb [dot] ac [dot] in)
Contact vishwas [at] cse iitb [dot] ac [dot] in

Tentative Topics

Definite topics (Arora–Barak Chs 1–7, plus some of Chs 8, 11)
Possible topics (material from Arora–Barak Chs 8, 13, 14, 16, 17, 19)

Grading

Quiz / Assignment
7 total, best 5 counted — every other Friday
25%
Mid-sem exam30%
End-sem exam45%
Paper presentation
Bonus
+20%

Assignments

1 Assignment 1 (PDF)
Tested in class on Fri, Aug 7, following a discussion / doubt-clearing session.

Lectures

Date # Topic
Tue, Jul 28 1
Introduction
Administrivia; problems of interest; Turing machines
Fri, Jul 31 2
P, NP, and reductions
Definitions of P and NP; polynomial-time reductions
Tue, Aug 4 3
Cook–Levin theorem and NP-hardness
Cook–Levin theorem; NP-hardness reductions
TBD 4
Diagonalization
Time and space hierarchy theorems, oracle separations
More lectures to be added as the course progresses.

References