Arithmetic progression

Sum

The sum of the members of a finite arithmetic progression is called an arithmetic series. For example, consider the sum:

2 + 5 + 8 + 11 + 14

This sum can be found quickly by taking the number n of terms being added (here 5), multiplying by the sum of the first and last number in the progression (here 2 + 14 = 16), and dividing by 2:

n ( a 1 + a n ) 2

In the case above, this gives the equation:

2 + 5 + 8 + 11 + 14 = 5 ( 2 + 14 ) 2 = 5 × 16 2 = 40.

This formula works for any real numbers a 1 and a n . For example:

( − 3 2 ) + ( − 1 2 ) + 1 2 = 3 ( − 3 2 + 1 2 ) 2 = − 3 2 .

Derivation

To derive the above formula, begin by expressing the arithmetic series in two different ways:

S n = a 1 + ( a 1 + d ) + ( a 1 + 2 d ) + ⋯ + ( a 1 + ( n − 2 ) d ) + ( a 1 + ( n − 1 ) d ) S n = ( a n − ( n − 1 ) d ) + ( a n − ( n − 2 ) d ) + ⋯ + ( a n − 2 d ) + ( a n − d ) + a n .

Adding both sides of the two equations, all terms involving d cancel:

  2 S n = n ( a 1 + a n ) .

Dividing both sides by 2 produces a common form of the equation:

S n = n 2 ( a 1 + a n ) .

An alternate form results from re-inserting the substitution: a n = a 1 + ( n − 1 ) d :

S n = n 2 [ 2 a 1 + ( n − 1 ) d ] .

Furthermore, the mean value of the series can be calculated via: S n / n :

n ¯ = a 1 + a n 2 .

In 499 AD Aryabhata, a prominent mathematician-astronomer from the classical age of Indian mathematics and Indian astronomy, gave this method in the Aryabhatiya (section 2.18).

Product

The product of the members of a finite arithmetic progression with an initial element a₁, common differences d, and n elements in total is determined in a closed expression

a 1 a 2 ⋯ a n = d a 1 d d ( a 1 d + 1 ) d ( a 1 d + 2 ) ⋯ d ( a 1 d + n − 1 ) = d n ( a 1 d ) n ¯ = d n Γ ( a 1 / d + n ) Γ ( a 1 / d ) ,

where x n ¯ denotes the rising factorial and Γ denotes the Gamma function. (Note however that the formula is not valid when a 1 / d is a negative integer or zero.)

This is a generalization from the fact that the product of the progression 1 × 2 × ⋯ × n is given by the factorial n ! and that the product

m × ( m + 1 ) × ( m + 2 ) × ⋯ × ( n − 2 ) × ( n − 1 ) × n

for positive integers m and n is given by

n ! ( m − 1 ) ! .

Taking the example from above, the product of the terms of the arithmetic progression given by a_n = 3 + (n-1)(5) up to the 50th term is

P 50 = 5 50 ⋅ Γ ( 3 / 5 + 50 ) Γ ( 3 / 5 ) ≈ 3.78438 × 10 98 .

Standard deviation

The standard deviation of any arithmetic progression can be calculated via:

σ = | d | ( n − 1 ) ( n + 1 ) 12

where n is the number of terms in the progression, and d is the common difference between terms

Intersections

The intersection of any two doubly infinite arithmetic progressions is either empty or another arithmetic progression, which can be found using the Chinese remainder theorem. If each two progressions in a family of doubly infinite arithmetic progressions have a non-empty intersection, then there exists a number common to all of them; that is, infinite arithmetic progressions form a Helly family. However, the intersection of infinitely many infinite arithmetic progressions might be a single number rather than itself being an infinite progression.

Formulas at a Glance

If

a 1 is the first term of an arithmetic progression. a n is the nth term of an arithmetic progression. d is the difference between terms of the arithmetic progression. n is the number of terms in the arithmetic progression. S n is the sum of n terms in the arithmetic progression. n ¯ is the mean value of arithmetic series.

then

1.   a n = a 1 + ( n − 1 ) d , 2.   a n = a m + ( n − m ) d . 3. S n = n 2 [ 2 a 1 + ( n − 1 ) d ] . 4. S n = n 2 ( a 1 + a n ) . 5. n ¯ = S n / n 6. n ¯ = a 1 + a n 2 .