Fiche de révision : Understanding Sequences and Series

📋 Course Outline

1. Sequence Definitions
2. Series Notation
3. Arithmetic Sequences
4. Geometric Sequences
5. Harmonic Sequences
6. Recursive Sequences
7. Finite Series
8. Infinite Series
9. Sequence Limits
10. Series Convergence
11. Convergence Tests
12. Harmonic Series Divergence

📖 1. Sequence Definitions

🔑 Key Concepts & Definitions

• Sequence: An ordered list of numbers ( a_n ), where each term corresponds to a specific position ( n ) (usually ( n \in \mathbb{N} )). It can be finite or infinite.
• Term: An individual element in a sequence, denoted as ( a_n ).
• Limit of a Sequence: The value ( L ) that ( a_n ) approaches as ( n \to \infty ), if such a limit exists.
• Convergence: A sequence converges if its terms approach a finite limit ( L ) as ( n ) increases indefinitely.
• Divergence: When a sequence does not approach a finite limit, it diverges (either tends to infinity, negative infinity, or oscillates).

📝 Essential Points

• Sequences are fundamental in calculus for defining limits, derivatives, and integrals.
• The notation ( a_n ) is standard for representing the ( n )-th term.
• Limits of sequences are used to analyze the behavior of functions and series.
• A sequence converges to ( L ) if, for every ( \epsilon > 0 ), there exists an ( N ) such that for all ( n > N ), ( |a_n - L| < \epsilon ).
• Common types include arithmetic (constant difference), geometric (constant ratio), and recursive sequences.

💡 Key Takeaway

Sequences are the building blocks for understanding limits and convergence in calculus; recognizing their types and limits is essential for analyzing the behavior of mathematical functions and series.

📖 2. Series Notation

🔑 Key Concepts & Definitions

• Series: The sum of the terms of a sequence, denoted as ( \sum_{k=1}^{n} a_k ) for finite sums or ( \sum_{k=1}^{\infty} a_k ) for infinite sums.

• Partial Sum: The sum of the first ( n ) terms of a series, written as ( S_n = \sum_{k=1}^{n} a_k ). It represents the accumulated total up to the ( n )-th term.

• Infinite Series: A series with an infinite number of terms, whose convergence depends on whether the sequence of partial sums ( S_n ) approaches a finite limit as ( n \to \infty ).

• Convergence of Series: An infinite series converges if the sequence of its partial sums ( S_n ) approaches a finite limit ( S ) as ( n \to \infty ); otherwise, it diverges.

• Summation Notation (Sigma notation): Compact way to represent the sum of a sequence's terms:

  [ \sum_{k=1}^{n} a_k ]

  where ( a_k ) is the ( k )-th term, and the limits indicate the start and end of the sum.

📝 Essential Points

• Series notation uses the sigma symbol ( \Sigma ) to denote summation over a sequence of terms.
• The partial sum ( S_n ) is crucial for analyzing the behavior of a series, especially for convergence.
• Infinite series are evaluated by examining the limit ( \lim_{n \to \infty} S_n ); if this limit exists and is finite, the series converges.
• The convergence or divergence of a series can be tested using various methods such as the divergence test, ratio test, and root test.
• Not all series converge; for example, the harmonic series ( \sum_{n=1}^{\infty} \frac{1}{n} ) diverges despite its terms approaching zero.

💡 Key Takeaway

Series notation provides a concise way to represent sums of sequences, and understanding the behavior of partial sums is essential for determining whether an infinite series converges or diverges.

📖 3. Arithmetic Sequences

🔑 Key Concepts & Definitions

• Arithmetic Sequence: A sequence of numbers where the difference between consecutive terms is constant, called the common difference ( d ).
  Form: ( a_n = a_1 + (n - 1)d )

• Common Difference (( d )): The fixed amount added to each term to get the next term in an arithmetic sequence.

• General Term (( a_n )): The expression that defines the ( n )-th term of the sequence, given by ( a_n = a_1 + (n - 1)d ).

• First Term (( a_1 )): The initial term of the sequence.

• Sum of First ( n ) Terms (( S_n )): The total of the first ( n ) terms, calculated as ( S_n = \frac{n}{2} (a_1 + a_n) ) or ( S_n = \frac{n}{2} [2a_1 + (n - 1)d] ).

📝 Essential Points

• The sequence progresses by adding a constant difference ( d ) each time.
• The ( n )-th term can be directly computed using ( a_n = a_1 + (n - 1)d ).
• The sum of the first ( n ) terms is efficiently calculated using the average of the first and last terms multiplied by ( n ).
• If ( d > 0 ), the sequence is increasing; if ( d < 0 ), it is decreasing.
• The sequence converges only if ( d = 0 ), in which case all terms are equal to ( a_1 ).

💡 Key Takeaway

An arithmetic sequence is characterized by a constant difference between terms, allowing for straightforward computation of any term and the sum of multiple terms, making it fundamental in understanding linear progressions and series.

📖 4. Geometric Sequences

🔑 Key Concepts & Definitions

• Geometric Sequence: A sequence where each term after the first is obtained by multiplying the previous term by a fixed constant called the common ratio ( r ).
  [ a_n = a_1 \cdot r^{n-1} ]

• Common Ratio (( r )): The constant factor between consecutive terms in a geometric sequence.
  [ r = \frac{a_{n+1}}{a_n} ]

• Finite Geometric Series: The sum of the first ( n ) terms of a geometric sequence.
  [ S_n = a_1 \frac{1 - r^n}{1 - r} \quad (r \neq 1) ]

• Infinite Geometric Series: The sum of infinitely many terms of a geometric sequence when ( |r| < 1 ).
  [ S_{\infty} = \frac{a_1}{1 - r} ]

• Convergence of Geometric Series: An infinite geometric series converges if and only if ( |r| < 1 ). If ( |r| \geq 1 ), the series diverges.

📝 Essential Points

• The general term ( a_n ) depends on the initial term ( a_1 ) and the common ratio ( r ).
• The sum of the first ( n ) terms (finite sum) is derived using the formula involving ( r^n ).
• Infinite geometric series converge only when the absolute value of the ratio ( r ) is less than 1; otherwise, they diverge.
• The formula for the sum of an infinite geometric series is valid only for ( |r| < 1 ).
• Geometric sequences are fundamental in modeling exponential growth or decay processes, such as population dynamics, radioactive decay, and finance.

💡 Key Takeaway

A geometric sequence is characterized by a constant ratio between terms, and its infinite sum converges only when the ratio's absolute value is less than one, making it a powerful tool for modeling exponential phenomena and summing infinite series under specific conditions.

📖 5. Harmonic Sequences

🔑 Key Concepts & Definitions

• Harmonic Sequence: A sequence where each term is the reciprocal of an integer, typically expressed as: [ a_n = \frac{1}{n} ] for ( n = 1, 2, 3, \ldots ).

• Harmonic Series: The infinite sum of the harmonic sequence: [ \sum_{n=1}^{\infty} \frac{1}{n} ] which is known to diverge.

• Divergence of Harmonic Series: The harmonic series does not converge to a finite limit; it diverges, meaning its partial sums grow without bound as ( n \to \infty ).

• Comparison Test: The harmonic series diverges because it can be compared to other divergent series, such as the integral of ( 1/x ).

📝 Essential Points

• The harmonic sequence ( a_n = 1/n ) approaches zero as ( n \to \infty ), but this does not imply the series converges.
• The harmonic series diverges logarithmically; its partial sums grow approximately like ( \ln n ).
• The divergence of the harmonic series is a fundamental example illustrating that terms tending to zero are not sufficient for series convergence.
• The Integral Test confirms divergence: since [ \int_{1}^{\infty} \frac{1}{x} dx = \infty, ] the harmonic series diverges.
• Variations of the harmonic series, such as ( \sum 1/n^p ), converge if and only if ( p > 1 ).

💡 Key Takeaway

The harmonic sequence decreases to zero, but its series diverges, illustrating that the terms of a series approaching zero is necessary but not sufficient for convergence. The harmonic series serves as a classic example of divergence in infinite series analysis.

📖 6. Recursive Sequences

🔑 Key Concepts & Definitions

• Recursive Sequence: A sequence where each term is defined in terms of one or more previous terms, along with initial conditions. It provides a rule to generate subsequent terms from earlier ones.

• Initial Terms: The first one or more terms of a recursive sequence that are given explicitly, serving as the starting point for generating further terms.

• Recurrence Relation: The formula that relates each term to previous terms, typically expressed as ( a_n = f(a_{n-1}, a_{n-2}, \ldots) ).

• Explicit Formula (Closed-Form): A non-recursive formula that directly computes the ( n )-th term without referencing previous terms, often derived from the recurrence relation.

• Homogeneous Recurrence Relation: A recurrence relation where the relation equals zero or a function of previous terms only, without additional non-recursive terms.

• Particular Solution: A specific solution to a non-homogeneous recurrence relation, often found using methods like undetermined coefficients.

📝 Essential Points

• Recursive sequences are defined by a recurrence relation and initial conditions; they are fundamental in modeling processes where each step depends on previous states (e.g., Fibonacci sequence).

• To analyze recursive sequences, find the recurrence relation and initial terms; then, attempt to derive an explicit formula for easier computation and analysis.

• Homogeneous linear recurrence relations with constant coefficients can be solved using characteristic equations, similar to solving differential equations.

• Non-homogeneous relations require finding a particular solution in addition to the homogeneous solution.

• Recursive sequences often appear in combinatorics, computer science (algorithm analysis), and mathematical modeling.

• Converting recursive definitions to explicit formulas simplifies calculations and helps analyze long-term behavior, such as limits and growth rates.

💡 Key Takeaway

Recursive sequences are defined by their recurrence relations and initial conditions, serving as powerful tools for modeling dependent processes; understanding how to solve and convert them into explicit formulas is essential for analyzing their behavior and applications.

📖 7. Finite Series

🔑 Key Concepts & Definitions

• Finite Series: The sum of a finite number of terms from a sequence, expressed as ( S_n = \sum_{k=1}^{n} a_k ), where ( n ) is a finite integer.
• Arithmetic Series: The sum of the first ( n ) terms of an arithmetic sequence, with a constant difference ( d ). Sum formula: ( S_n = \frac{n}{2} (a_1 + a_n) ).
• Geometric Series: The sum of the first ( n ) terms of a geometric sequence with ratio ( r ). Sum formula: ( S_n = a_1 \frac{1 - r^n}{1 - r} ) (for ( r \neq 1 )).
• Sum of a Series: The total obtained by adding all terms in the series, denoted as ( S_n ) for finite sums.
• Partial Sum: The sum of the first ( n ) terms of a series, used to analyze the behavior of the series as ( n ) increases.

📝 Essential Points

• Finite series are straightforward to evaluate using explicit formulas, unlike infinite series.
• The sum of an arithmetic series depends on the first and last terms, or the first term and common difference.
• Geometric series sum formulas are valid for any finite ( n ), and the series converges only when considering the infinite case with ( |r| < 1 ).
• When calculating sums, always verify the type of series (arithmetic or geometric) to apply the correct formula.
• Finite series are foundational for understanding the behavior of their infinite counterparts and for practical applications where only a limited number of terms are involved.

💡 Key Takeaway

Finite series allow for exact calculation of sums using specific formulas, serving as essential building blocks for understanding more complex infinite series and their convergence properties.

📖 8. Infinite Series

🔑 Key Concepts & Definitions

• Infinite Series: The sum of infinitely many terms of a sequence, expressed as ( \sum_{n=1}^{\infty} a_n ). It converges if the sequence of partial sums approaches a finite limit.

• Partial Sum (( S_n )): The sum of the first ( n ) terms of a series, ( S_n = \sum_{k=1}^{n} a_k ). The behavior of ( S_n ) as ( n \to \infty ) determines convergence.

• Convergence: An infinite series converges if its partial sums ( S_n ) approach a finite limit ( S ) as ( n \to \infty ). Otherwise, it diverges.

• Divergence: An infinite series diverges if the partial sums do not approach a finite limit, often indicated by the limit of ( a_n ) not being zero.

• Geometric Series: A series where each term is multiplied by a common ratio ( r ), ( \sum_{n=0}^{\infty} ar^n ). It converges if ( |r| < 1 ), with sum ( \frac{a}{1 - r} ).

• Test for Convergence (Ratio Test): For ( a_n > 0 ), compute ( L = \lim_{n \to \infty} \left| \frac{a_{n+1}}{a_n} \right| ). If ( L < 1 ), the series converges; if ( L > 1 ), it diverges; if ( L = 1 ), the test is inconclusive.

📝 Essential Points

• Necessary Condition for Convergence: ( a_n \to 0 ) as ( n \to \infty ). If ( \lim_{n \to \infty} a_n \neq 0 ), the series diverges (Divergence Test).

• Geometric Series: Converges only when ( |r| < 1 ). The sum is ( \frac{a}{1 - r} ).

• Harmonic Series: ( \sum_{n=1}^{\infty} \frac{1}{n} ) diverges despite terms approaching zero, illustrating that ( a_n \to 0 ) is necessary but not sufficient for convergence.

• Power Series: Series of the form ( \sum a_n (x - c)^n ). Convergence depends on the radius of convergence ( R ), which can be found using the Ratio or Root Test.

• Common Tests for Series:

  • Comparison Test: Compare with a known convergent or divergent series.
  • Limit Comparison Test: Uses the limit of the ratio of terms.
  • Integral Test: Uses integrals to test convergence for positive, decreasing ( a_n ).

💡 Key Takeaway

Infinite series converge only when their partial sums approach a finite limit, with geometric series providing a fundamental example where convergence depends on the ratio ( r ). Recognizing divergence often hinges on the behavior of individual terms and applying appropriate convergence tests.

📖 9. Sequence Limits

🔑 Key Concepts & Definitions

• Limit of a Sequence: The value ( L ) that the terms ( a_n ) of a sequence approach as ( n \to \infty ). Denoted as: [ \lim_{n \to \infty} a_n = L ] if for every ( \epsilon > 0 ), there exists ( N ) such that for all ( n > N ), ( |a_n - L| < \epsilon ).

• Convergent Sequence: A sequence whose terms approach a finite limit ( L ) as ( n \to \infty ).

• Divergent Sequence: A sequence that does not approach a finite limit; it may diverge to infinity or oscillate indefinitely.

• Limit Laws: Rules that allow the computation of limits involving sums, products, and quotients of sequences, such as: [ \lim_{n \to \infty} (a_n \pm b_n) = \lim_{n \to \infty} a_n \pm \lim_{n \to \infty} b_n ] (when these limits exist).

• Squeeze Theorem: If ( a_n \leq b_n \leq c_n ) for all ( n ) beyond some ( N ), and [ \lim_{n \to \infty} a_n = \lim_{n \to \infty} c_n = L, ] then [ \lim_{n \to \infty} b_n = L. ]

📝 Essential Points

• To determine the limit of a sequence, analyze its behavior as ( n \to \infty ), often using algebraic manipulation, known limits, or the squeeze theorem.
• Sequences like ( a_n = \frac{1}{n} ) tend to 0, illustrating convergence to a finite limit.
• Sequences such as ( a_n = n ) diverge to infinity.
• Limits are fundamental in defining the convergence of series and in calculus operations like differentiation and integration.
• For sequences involving exponential, polynomial, or logarithmic functions, apply limit laws, L'Hôpital's rule, or known standard limits.

💡 Key Takeaway

The limit of a sequence describes its long-term behavior; understanding how to evaluate these limits is essential for analyzing convergence and the behavior of functions in calculus.

📖 10. Series Convergence

🔑 Key Concepts & Definitions

• Convergent Series: An infinite series ( \sum_{n=1}^\infty a_n ) is said to converge if its sequence of partial sums ( S_N = \sum_{n=1}^N a_n ) approaches a finite limit as ( N \to \infty ).

• Divergent Series: An infinite series that does not approach a finite limit; the partial sums either grow without bound or oscillate indefinitely.

• Limit of a Series: The value ( S ) that the partial sums ( S_N ) approach as ( N \to \infty ). If this limit exists and is finite, the series converges to ( S ).

• Test for Convergence (Divergence Test): If ( \lim_{n \to \infty} a_n \neq 0 ), then ( \sum a_n ) diverges. The converse is not necessarily true; ( a_n \to 0 ) does not guarantee convergence.

• Ratio Test: For ( a_n > 0 ), compute ( L = \lim_{n \to \infty} \left| \frac{a_{n+1}}{a_n} \right| ):

  • If ( L < 1 ), the series converges absolutely.
  • If ( L > 1 ), the series diverges.
  • If ( L = 1 ), the test is inconclusive.

• Root Test: For ( a_n \geq 0 ), compute ( L = \lim_{n \to \infty} \sqrt[n]{a_n} ):

  • If ( L < 1 ), series converges absolutely.
  • If ( L > 1 ), series diverges.
  • If ( L = 1 ), the test is inconclusive.

📝 Essential Points

• Convergence depends on the behavior of partial sums: The key is whether ( S_N ) approaches a finite limit as ( N \to \infty ).

• Geometric series: Converges if ( |r| < 1 ), with sum ( \frac{a}{1 - r} ); diverges otherwise.

• Harmonic series: ( \sum_{n=1}^\infty \frac{1}{n} ) diverges, despite terms tending to zero.

• Comparison tests: Series with smaller terms than a convergent series also converge; larger terms than a divergent series also diverge.

• Absolute vs. conditional convergence: A series converges absolutely if ( \sum |a_n| ) converges; otherwise, it may converge conditionally.

💡 Key Takeaway

A series converges only if its partial sums approach a finite limit; convergence can be tested using various criteria such as the Ratio and Root Tests, but the divergence of the terms ( a_n ) to zero is a necessary, not sufficient, condition for convergence.

📖 11. Convergence Tests

🔑 Key Concepts & Definitions

• Convergence of Series: An infinite series ( \sum a_n ) converges if its sequence of partial sums ( S_n = \sum_{k=1}^n a_k ) approaches a finite limit as ( n \to \infty ).

• Divergence Test (Term Test): If ( \lim_{n \to \infty} a_n \neq 0 ), then the series ( \sum a_n ) diverges. Conversely, if ( a_n \to 0 ), the test is inconclusive.

• Ratio Test: For ( a_n > 0 ), compute ( L = \lim_{n \to \infty} \frac{a_{n+1}}{a_n} ).

  • If ( L < 1 ), the series converges absolutely.
  • If ( L > 1 ), the series diverges.
  • If ( L = 1 ), the test is inconclusive.

• Root Test: For ( a_n \geq 0 ), compute ( L = \lim_{n \to \infty} \sqrt[n]{a_n} ).

  • If ( L < 1 ), the series converges absolutely.
  • If ( L > 1 ), the series diverges.
  • If ( L = 1 ), the test is inconclusive.

• Comparison Test: If ( 0 \leq a_n \leq b_n ) for all ( n ), and ( \sum b_n ) converges, then ( \sum a_n ) converges. If ( \sum b_n ) diverges and ( a_n \geq b_n ), then ( \sum a_n ) diverges.

📝 Essential Points

• The Divergence Test is a quick initial check; if the terms do not tend to zero, the series cannot converge.
• The Ratio and Root Tests are especially useful for series with factorials, exponentials, or roots.
• The Comparison Test helps determine convergence by comparing to known series (like geometric or p-series).
• The Limit Comparison Test is a variation that compares the limit of ( a_n / b_n ) to a finite, non-zero constant.
• For series with alternating signs, the Alternating Series Test states that if the absolute value of the terms decreases monotonically to zero, the series converges.

💡 Key Takeaway

Convergence tests are essential tools for analyzing infinite series; selecting the appropriate test depends on the series' form, and often multiple tests are used in combination to establish convergence or divergence.

📖 12. Harmonic Series Divergence

🔑 Key Concepts & Definitions

• Harmonic Series: An infinite series of the form ( \sum_{n=1}^{\infty} \frac{1}{n} ), summing the reciprocals of natural numbers.

• Divergence: A series diverges if its sequence of partial sums does not approach a finite limit as ( n \to \infty ).

• Partial Sums: The sum of the first ( n ) terms of a series, denoted ( S_n = \sum_{k=1}^{n} a_k ).

• Comparison Test: A method to determine divergence by comparing a series to another series known to diverge or converge.

• Integral Test: A test for convergence/divergence involving the integral of a related function; for ( a_n = f(n) ), if ( f ) is positive, decreasing, then ( \sum a_n ) and ( \int f(x) dx ) share the same convergence behavior.

📝 Essential Points

• The harmonic series ( \sum_{n=1}^{\infty} \frac{1}{n} ) diverges, meaning its partial sums grow without bound.

• Despite the terms ( \frac{1}{n} ) tending to zero, this is not sufficient for convergence; the series still diverges.

• Proof of divergence can be demonstrated via the Comparison Test by comparing the harmonic series to a series with known divergence or through the Integral Test:

  [ \int_1^{\infty} \frac{1}{x} dx = \lim_{t \to \infty} \ln t = \infty ]

  Since the integral diverges, the harmonic series diverges.

• The divergence of the harmonic series is a fundamental example illustrating that terms tending to zero do not guarantee convergence.

• The divergence persists even when the terms decrease very slowly, highlighting the importance of convergence tests beyond the limit of terms.

💡 Key Takeaway

The harmonic series diverges despite its terms approaching zero, illustrating that the necessary condition for convergence (terms tending to zero) is not sufficient; additional tests like the Integral Test confirm its divergence.

📊 Synthesis Tables

⚠️ Common Pitfalls & Confusions

1. Assuming a sequence converges just because ( a_n \to 0 ).
2. Confusing the convergence of a sequence with the convergence of a series.
3. Using the formula for the sum of a finite geometric series when ( r=1 ).
4. Believing all series with terms approaching zero are convergent.
5. Overlooking the divergence of the harmonic series despite its terms tending to zero.
6. Misapplying the geometric series sum formula outside ( |r| < 1 ).
7. Confusing recursive definitions with explicit formulas for sequences.

✅ Exam Checklist

• Recall the definition of a sequence and the meaning of convergence and divergence.
• Know the notation for series ( \sum_{k=1}^n a_k ) and partial sums ( S_n ).
• Be able to derive the explicit formula for arithmetic sequences and sums.
• Understand the general form of geometric sequences and the conditions for series convergence.
• Recognize the divergence of the harmonic series and the significance of terms tending to zero.
• Apply convergence tests such as the divergence test, ratio test, and root test.
• Differentiate between finite and infinite series and their convergence criteria.
• Calculate the sum of finite geometric series and the sum of infinite geometric series when applicable.
• Identify whether a sequence or series converges or diverges based on its properties.
• Use the integral test to determine divergence of series like the harmonic series.
• Understand the behavior of harmonic sequences and series, including their divergence.