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Summary - (1/2) Stochastic Modelling (X_400646)

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Preparing for the Stochastic Modelling midterm at VU Amsterdam? It's no secret—it's tough! But, breathe easy. This summary has you covered for the first midterm. Don't drown in the sea of notes, streamline your revision with this guide.

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  • October 17, 2023
  • 16
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Contents
1 Introduction to Markov Chains 3
1.1 Why Markov Chains? . . . . . . . . . . . . . . . . . . . . . . . . 3
1.2 Characteristics of Discrete-time Markov Chains . . . . . . . . . . 3
1.2.1 Discrete-time . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.2.2 Countable State Space . . . . . . . . . . . . . . . . . . . . 3
1.2.3 Markov Chains . . . . . . . . . . . . . . . . . . . . . . . . 3
1.2.4 Time-homogeneity . . . . . . . . . . . . . . . . . . . . . . 3
1.3 Transition Diagram and Matrix . . . . . . . . . . . . . . . . . . . 3
1.3.1 Transition Diagram . . . . . . . . . . . . . . . . . . . . . 3
1.3.2 Transition Matrix . . . . . . . . . . . . . . . . . . . . . . 4
1.4 Formulating a DTMC . . . . . . . . . . . . . . . . . . . . . . . . 4

2 Examples and Remarks 4
2.1 Gambling or Random Walk . . . . . . . . . . . . . . . . . . . . . 4
2.2 Sanity Check . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

3 Conclusion 5

4 Transient Distribution 6
4.1 Probability to Visit a Sequence of States . . . . . . . . . . . . . . 6
4.2 n-step Transition Probabilities . . . . . . . . . . . . . . . . . . . 6
4.3 Transient Distribution at Time n . . . . . . . . . . . . . . . . . . 6

5 Hitting Times 7
5.1 Hitting Times and Probabilities . . . . . . . . . . . . . . . . . . . 7
5.2 Expectation of Hitting Times . . . . . . . . . . . . . . . . . . . . 7
5.3 Conditioning on the First Step . . . . . . . . . . . . . . . . . . . 7
5.4 Remarks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

6 Communicating Classes in Markov Chains 8
6.1 Definition and Properties . . . . . . . . . . . . . . . . . . . . . . 8
6.1.1 Definition (Communicating States): . . . . . . . . . 8
6.1.2 Definition (Absorbing Class): . . . . . . . . . . . . . . . . 8
6.1.3 Definition (Periodicity): . . . . . . . . . . . . . . . . . . . 8
6.1.4 Definition (Irreducibility): . . . . . . . . . . . . . . . . . . 8

7 Long-term Behavior of DTMCs 9
7.1 Definitions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
7.2 Practical Importance . . . . . . . . . . . . . . . . . . . . . . . . . 9
7.2.1 π occ : . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
7.2.2 π lim : . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9




1

, 7.3 Finding π occ and π lim . . . . . . . . . . . . . . . . . . . . . . . . 10
7.4 Existence π lim , π occ . . . . . . . . . . . . . . . . . . . . . . . . . 10
7.5 Strategy for Multiclass . . . . . . . . . . . . . . . . . . . . . . . . 10

8 Costs in DTMCs 11
8.1 Long-run Average Costs . . . . . . . . . . . . . . . . . . . . . . . 11
8.2 Expected Costs in Equilibrium . . . . . . . . . . . . . . . . . . . 11

9 Exponential Distribution 12
9.1 Memorylessness . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
9.2 Total Probability Law . . . . . . . . . . . . . . . . . . . . . . . . 13
9.3 Competing Exponentials . . . . . . . . . . . . . . . . . . . . . . . 13

10 The Poisson Process 14
10.1 Context: . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
10.2 Definition 1 (Exponential inter-arrivals): . . . . . . . . . . . . . . 14
10.3 Definition 2 (Poisson increments): . . . . . . . . . . . . . . . . . . 14
10.4 Remark on Poisson process vs. Poisson distribution: . . . . . . . 15
10.5 Properties: . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
10.6 Interconnection of Poisson Process Properties: . . . . . . . . . . . 15

11 Merging & Splitting for Poisson Random Variables 16
11.1 Definition of Poisson Random Variable . . . . . . . . . . . . . . . 16
11.2 Merging of Poisson Random Variables . . . . . . . . . . . . . . . 16
11.3 Splitting / Thinning of Poisson Random Variables . . . . . . . . 16
11.4 Merging & Splitting for Poisson Processes . . . . . . . . . . . . . 16
11.4.1 Merging of Poisson Processes . . . . . . . . . . . . . . . . 16
11.4.2 Splitting / Thinning of Poisson Processes . . . . . . . . . 17




2

, 1 Introduction to Markov Chains
1.1 Why Markov Chains?
• Markov Chains are tractable: analysis is feasible with many standard
procedures.
• They can predict time-average behavior, such as average costs per day or
the fraction of lost customers.
• Many real-world scenarios can be modeled using Markov Chains.

1.2 Characteristics of Discrete-time Markov Chains
1.2.1 Discrete-time
A DTMC is a sequence of random variables {X0 , X1 , X2 , . . .} where the index
n represents discrete time units.

1.2.2 Countable State Space
All possible values of Xn for n = 0, 1, 2, . . . are in a countable set S, termed the
state space.

1.2.3 Markov Chains
A sequence of random variables is a Markov Chain if it possesses the Markov
property: the next state depends only on the present state, not on any prior
history.

1.2.4 Time-homogeneity
A DTMC is time-homogeneous if its transition probabilities remain constant
over time.

1.3 Transition Diagram and Matrix
1.3.1 Transition Diagram
The transition diagram is a graphical representation of a DTMC. It provides a
visual overview of the system’s dynamics, making it easier to understand and
analyze.

• States: Each state in the state space S is represented as a node or circle
in the diagram.
• Transitions: Arrows between states represent possible transitions. The
direction of the arrow indicates the direction of the transition.




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