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IB DP Maths AA SL Study Notes

1.2.1 Introduction to Sigma Notation

Definition of Sigma Notation

Sigma notation is a mathematical shorthand that provides a systematic and concise way to represent the sum of terms in a sequence or series. It is especially useful when dealing with long sums, allowing them to be written in a more compact and manageable form. For a deeper understanding, you can explore the basics of arithmetic sequences which are closely related.

Basic Form

The general form of sigma notation is expressed as:

S = Σ (from n=a to b) f(n)

  • S represents the sum of the sequence.
  • n is the index of summation, indicating the variable of summation.
  • a and b are the lower and upper bounds of the sum, respectively.
  • f(n) is the expression to be summed, where f is a function that generates terms of the sequence.

Example 1

Consider the sum of the first five positive integers:

S = 1 + 2 + 3 + 4 + 5

Using sigma notation, this sum can be written as:

S = Σ (from n=1 to 5) n

In this example, n takes on values from 1 to 5, and each value is summed to obtain the total S.

Properties of Sigma Notation

Sigma notation is not just a symbolic representation but also possesses algebraic properties that allow for the manipulation and simplification of expressions involving sums. These properties are essential for evaluating sums and performing algebraic operations on them. To explore more complex uses, visit our section on advanced sigma notation.

Property 1: Constants Outside the Sum

If a sum involves a constant multiplier within the function being summed, this constant can be taken outside of the sigma notation, simplifying the expression.

Σ (from n=a to b) cf(n) = c Σ (from n=a to b) f(n)

Property 2: Sum of Sums

The sum of two sequences can be separated into two distinct sums, allowing for separate evaluation and simplification.

Σ (from n=a to b) [f(n) + g(n)] = Σ (from n=a to b) f(n) + Σ (from n=a to b) g(n)

Property 3: Shifting the Index

Adjusting the index of summation does not alter the sum if the adjustment is performed correctly.

Σ (from n=a to b) f(n) = Σ (from n=a+k to b+k) f(n-k)

Example 2

Evaluate the sum:

S = Σ (from n=2 to 5) 3n

Using property 1, we can take the constant 3 outside of the sum:

S = 3 Σ (from n=2 to 5) n

Now, we substitute the values of n from 2 to 5 and add them up:

S = 3 x (2 + 3 + 4 + 5) S = 3 x 14 S = 42

Basic Summation Examples

Arithmetic Series

An arithmetic series is the sum of the terms of an arithmetic sequence, a sequence of numbers in which the difference between consecutive terms is constant. This difference could be positive, negative, or zero. For more detail on this concept, refer to our notes on the basics of arithmetic sequences.

The sum of an arithmetic series can be found using the formula:

S = n/2 [2a + (n-1)d]

Where:

  • S is the sum of the series.
  • n is the number of terms.
  • a is the first term.
  • d is the common difference between the terms.

Example 3

Find the sum of the first 100 positive integers.

Using the formula for the sum of an arithmetic series and the sigma notation:

S = Σ (from n=1 to 100) n

S = n/2 [2a + (n-1)d]

Substituting n = 100, a = 1, and d = 1 (since the common difference between consecutive integers is 1), we get:

S = 100/2 [2 x 1 + (100-1) x 1] S = 50 x [2 + 99] S = 50 x 101 S = 5050

Geometric Series

A geometric series is the sum of the terms of a geometric sequence, where each term after the first is found by multiplying the previous term by a fixed, non-zero number called the ratio.

The sum S of the first n terms of a geometric series is calculated as:

S = a(1-rn) / (1-r)

Where:

  • a is the first term.
  • r is the common ratio.
  • n is the number of terms.

Example 4

Find the sum of the first 5 terms of the geometric series: 3, 6, 12, 24, 48, ...

Using the formula for the sum of a geometric series:

S = Σ (from n=1 to 5) 3 x 2(n-1)

S = 3(1-25) / (1-2)

S = 3(1-32) / -1

S = 3 x -31 / -1

S = 93

In these notes, we have explored the definition, properties, and basic summation examples of sigma notation, providing a foundational understanding for IB Mathematics students. Through the examples provided, students can observe the application of sigma notation and its properties in solving problems related to arithmetic and geometric series, enhancing their problem-solving skills in various mathematical contexts. For further insights, you might want to look into exponential equations and proof by mathematical induction. This foundational knowledge will be instrumental in navigating through more complex mathematical concepts and applications in the IB Mathematics curriculum.

FAQ

Yes, sigma notation has a counterpart for products, known as pi notation, which is represented by the Greek letter Pi (Π). Similar to sigma notation, pi notation provides a concise way to express the product of a sequence of terms. The general form is Π (from n=a to b) f(n), where n is the index of multiplication, a and b are the lower and upper bounds, respectively, and f(n) is the expression to be multiplied. Pi notation is particularly useful in number theory and combinatorics, where products of sequences often arise, and it provides a systematic and compact way to represent them.

Sigma notation and limits in calculus are interconnected, especially in the context of series and sequences. When dealing with infinite series, represented with sigma notation, we often explore the concept of limits to determine the convergence or divergence of the series. Specifically, we examine the limit of the partial sums of the series as the number of terms approaches infinity. If the limit exists and is finite, the series is said to converge; otherwise, it diverges. Thus, sigma notation provides a framework for expressing series, while limits offer a tool for analysing their behaviour, particularly in the realm of infinite series.

Absolutely, sigma notation can be used alongside various mathematical notations and operations, including integrals in calculus. For instance, in the context of Riemann sums, sigma notation is used to express the sum of the areas of rectangles under a curve, which approximates the definite integral of a function. The limit of these Riemann sums as the width of the rectangles approaches zero is equal to the definite integral of the function over the interval. Thus, sigma notation plays a pivotal role in connecting discrete sums with continuous integrals, facilitating the exploration and understanding of integral calculus.

Yes, sigma notation can be used to represent infinite series by using infinity (∞) as the upper limit of summation. In this context, the notation Σ (from n=a to ∞) f(n) represents the sum of the terms f(n) from n equals a to infinity. It's crucial to note that not all infinite series have a finite sum. The sum of an infinite series is defined as the limit of the partial sums (if it exists) as the number of terms approaches infinity. Convergence tests, such as the ratio test or the comparison test, are often used to determine whether an infinite series converges to a finite value or diverges.

Sigma notation significantly streamlines the expression of long sums in mathematical proofs and derivations by providing a concise and systematic representation. Instead of writing out each term of a sum individually, which can be impractical or impossible for large sums or infinite series, sigma notation encapsulates the entire sum in a single expression. This not only makes mathematical expressions more readable and manageable but also facilitates algebraic manipulations within proofs and derivations. The use of sigma notation allows mathematicians and students alike to work with sums in a more general and abstract form, enabling the development and proof of formulas and theorems that can be applied to specific cases as needed.

Practice Questions

Evaluate the sum of the first 10 terms of the arithmetic series where the first term is 3 and the common difference is 2.

The sum of an arithmetic series can be found using the formula: S = n/2 [2a + (n-1)d] where:

  • S is the sum of the series,
  • n is the number of terms,
  • a is the first term, and
  • d is the common difference.

Substituting n = 10, a = 3, and d = 2 into the formula, we get: S = 10/2 [2 * 3 + (10-1) * 2] S = 5 * [6 + 18] S = 5 * 24 S = 120

Therefore, the sum of the first 10 terms of the arithmetic series is 120.

Determine the sum of the geometric series: 4, 12, 36, ..., for the first 6 terms.

The sum S of the first n terms of a geometric series is calculated using the formula: S = a(1-rn) / (1-r) where:

  • a is the first term,
  • r is the common ratio, and
  • n is the number of terms.

In this series, a = 4 and r = 3. We are asked to find the sum of the first 6 terms, so n = 6. Substituting these values into the formula, we get: S = 4(1-36) / (1-3) S = 4(1-729) / -2 S = 4 * -728 / -2 S = 4 * 364 S = 1456

Thus, the sum of the first 6 terms of the geometric series is 1456.

Dr Rahil Sachak-Patwa avatar
Written by: Dr Rahil Sachak-Patwa
LinkedIn
Oxford University - PhD Mathematics

Rahil spent ten years working as private tutor, teaching students for GCSEs, A-Levels, and university admissions. During his PhD he published papers on modelling infectious disease epidemics and was a tutor to undergraduate and masters students for mathematics courses.

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