Leibniz's Formula for Pi
Theorem
- $\dfrac \pi 4 = 1 - \dfrac 1 3 + \dfrac 1 5 - \dfrac 1 7 + \dfrac 1 9 - \cdots \approx 0 \cdotp 78539 \, 81633 \, 9744 \ldots$
This sequence is A003881 in the On-Line Encyclopedia of Integer Sequences (N. J. A. Sloane (Ed.), 2008).
That is:
- $\ds \pi = 4 \sum_{k \mathop \ge 0} \paren {-1}^k \frac 1 {2 k + 1}$
Elementary Proof
First we note that:
- $(1): \quad \dfrac 1 {1 + t^2} = 1 - t^2 + t^4 - t^6 + \cdots + t^{4 n} - \dfrac {t^{4 n + 2} } {1 + t^2}$
which is demonstrated here.
Now consider the real number $x \in \R: 0 \le x \le 1$.
We can integrate expression $(1)$ with respect to $t$ from $0$ to $x$:
- $(2): \quad \ds \int_0^x \frac {\d t} {1 + t^2} = x - \frac {x^3} 3 + \frac {x^5} 5 - \frac {x^7} 7 + \cdots + \frac {x^{4 n + 1} } {4 n + 1} - \map {R_n} x$
where:
- $\ds \map {R_n} x = \int_0^x \frac {t^{4 n + 2} } {1 + t^2} \rd t$
From Square of Real Number is Non-Negative we have that:
- $t^2 \ge 0$
and so:
- $1 \le 1 + t^2$
From Relative Sizes of Definite Integrals, we have:
- $\ds 0 \le \map {R_n} x \le \int_0^x t^{4 n + 2} \rd t$
that is:
- $0 \le \map {R_n} x \le \dfrac {x^{4n + 3} } {4 n + 3}$
But as $0 \le x \le 1$ it is clear that:
- $\dfrac {x^{4 n + 3} } {4 n + 3} \le \dfrac 1 {4 n + 3}$
So:
- $0 \le \map {R_n} x \le \dfrac 1 {4 n + 3}$
From Basic Null Sequences and the Squeeze Theorem, $\dfrac 1 {4 n + 3} \to 0$ as $n \to \infty$.
This leads us directly to:
- $(3): \quad \ds \int_0^x \frac {\d t} {1 + t^2} = x - \frac {x^3} 3 + \frac {x^5} 5 - \frac {x^7} 7 + \frac {x^9} 9 \cdots$
But from Derivative of Arctangent Function, we also have that:
- $\dfrac \d {\d x} \arctan t = \dfrac 1 {1 + t^2}$
and thence from the Fundamental Theorem of Calculus we have:
- $\ds \arctan x = \int_0^x \frac {\d t} {1 + t^2}$
From $(3)$ it follows immediately that:
- $(4): \quad \arctan x = x - \dfrac {x^3} 3 + \dfrac {x^5} 5 - \dfrac {x^7} 7 + \dfrac {x^9} 9 \cdots$
Now all we need to do is plug $x = 1$ into $(4)$.
$\blacksquare$
Leibniz's Proof
The area $OAT$ is a quarter-circle whose area is $\dfrac \pi 4$ by Area of Circle.
Now consider the area $C$ of the segment $OPQT$, bounded by the arc $OT$ and the chord $OT$.
Consider the area $OPQ$, bounded by the line segments $OP$ and $OQ$ and the arc $PQ$.
As $P$ and $Q$ approach each other, the arc $PQ$ tends towards the straight line segment $\d s = PQ$.
We can therefore consider the area $OPQ$ as a triangle.
We extend the line segment $PQ$ and drop a perpendicular $OR$ to $O$.
Using Area of Triangle in Terms of Side and Altitude, we see that the area $\d C$ of $\triangle OPQ$ is given by:
- $\d C = \triangle OPQ = \dfrac {OR \cdot PQ} 2 = \dfrac {OR \cdot \d s} 2$
We also note from elementary Euclidean geometry that $\triangle ORS$ is similar to the small triangle on $PQ$.
Thus:
- $\dfrac {\d s} {\d x} = \dfrac {OS} {OR} \iff OR \cdot \d s = OS \cdot \d x$
Thus:
- $\d C = \dfrac {OS \cdot \d x} 2 = \dfrac {y \rd x} 2$
where $y = OS$.
We set the horizontal coordinate of $P$ as equal to $x$.
Thus the total area $C$ is equal to the total of all the areas of these small triangles as $x$ increases from $0$ to $1$.
So:
- $\ds C = \int \rd C = \frac 1 2 \int_0^1 y \rd x$
Now we use Integration by Parts to swap $x$ and $y$:
- $\ds C = \intlimits {\frac 1 2 x y} 0 1 - \frac 1 2 \int_0^1 x \rd y = \frac 1 2 - \frac 1 2 \int_0^1 x \rd y$
It can be seen that the limits on this new integral have to be $0$ and $1$ from the geometry of the situation.
Now we note that:
- $y = \tan \dfrac \phi 2$
From Double Angle Formula for Cosine: Corollary $2$:
- $x = 1 - \cos \phi = 2 \sin^2 \dfrac \phi 2$
Thus:
\(\ds \tan^2 \frac \phi 2\) | \(=\) | \(\ds \frac {\sin^2 \frac \phi 2}{\cos^2 \frac \phi 2}\) | Tangent is Sine divided by Cosine | |||||||||||
\(\ds \) | \(=\) | \(\ds \sin^2 \frac \phi 2 \sec^2 \frac \phi 2\) | Secant is Reciprocal of Cosine | |||||||||||
\(\ds \) | \(=\) | \(\ds \sin^2 \frac \phi 2 \paren {1 + \tan^2 \frac \phi 2}\) | Difference of Squares of Secant and Tangent |
and so:
- $\dfrac x 2 = \dfrac {y^2} {1 + y^2}$
Using Sum of Infinite Geometric Sequence:
- $\dfrac {y^2} {1 + y^2} = y^2 - y^4 + y^6 - y^8 + \cdots$
This gives us:
\(\ds C\) | \(=\) | \(\ds \frac 1 2 - \int_0^1 \paren {y^2 - y^4 + y^6 - y^8 + \cdots} \d y\) | ||||||||||||
\(\ds \) | \(=\) | \(\ds \frac 1 2 - \intlimits {\frac {y^3} 3 - \frac {y^5} 5 + \frac {y^7} 7 - \frac {y^9} 9 + \cdots} 0 1\) | ||||||||||||
\(\ds \) | \(=\) | \(\ds \frac 1 2 - \paren {\frac 1 3 - \frac 1 5 + \frac 1 7 - \frac 1 9 + \cdots}\) | ||||||||||||
\(\ds \) | \(=\) | \(\ds \frac 1 2 - \frac 1 3 + \frac 1 5 - \frac 1 7 + \frac 1 9 - \cdots\) |
Remember, $C$ is the area of the segment $OPQT$.
Now we add to it the area of $\triangle OTA$, which trivially equals $\dfrac 1 2$, to get the area of the quarter circle which we know as equal to $\dfrac \pi 4$.
Putting it all together, this gives us:
- $\dfrac \pi 4 = 1 - \dfrac 1 3 + \dfrac 1 5 - \dfrac 1 7 + \dfrac 1 9 - \cdots$
$\blacksquare$
Proof by Taylor Expansion
From Power Series Expansion for Real Arctangent Function, we obtain:
- $\arctan x = x - \dfrac {x^3} 3 + \dfrac {x^5} 5 - \dfrac {x^7} 7 + \dfrac {x^9} 9 - \cdots$
Substituting $x = 1$ gives the required result.
$\blacksquare$
Proof by Dirichlet Beta Function
Recall the Dirichlet beta function:
- $\ds \map \beta s = \sum_{n \mathop = 0}^\infty \frac {\paren {-1}^n} {\paren {2 n + 1}^s}$
From Dirichlet Beta Function at Odd Positive Integers, we obtain:
- $\map \beta {2 n + 1} = \paren {-1}^n \dfrac {E_{2 n} \pi^{2 n + 1} } {4^{n + 1} \paren {2 n}!}$
Therefore, setting $n = 0$ above:
\(\ds \map \beta 1\) | \(=\) | \(\ds \sum_{n \mathop = 0}^\infty \frac {\paren {-1}^n} {\paren {2 n + 1}^1}\) | Definition of Dirichlet Beta Function | |||||||||||
\(\ds \) | \(=\) | \(\ds \paren {-1}^0 \dfrac {E_0 \pi } {4 \times 0!}\) | Dirichlet Beta Function at Odd Positive Integers | |||||||||||
\(\ds \) | \(=\) | \(\ds \frac \pi 4\) | Definition of Euler Numbers and Factorial of Zero |
$\blacksquare$
Proof by Digamma Function
\(\ds 2 b \sum_{k \mathop = 1}^\infty \dfrac {\paren {-1}^{k + 1} } {a + b k}\) | \(=\) | \(\ds \map \psi {\dfrac a {2 b} + 1} - \map \psi {\dfrac a {2 b} + \dfrac 1 2}\) | Reciprocal times Derivative of Gamma Function: Corollary $2$ | |||||||||||
\(\ds \leadsto \ \ \) | \(\ds -4 \sum_{k \mathop = 1}^\infty \dfrac {\paren {-1}^k} {1 + 2 k}\) | \(=\) | \(\ds \map \psi {\dfrac 1 4 + 1} - \map \psi {\dfrac 1 4 + \dfrac 1 2}\) | $a := 1$ and $b := 2$ | ||||||||||
\(\ds \leadsto \ \ \) | \(\ds 4 \sum_{k \mathop = 1}^\infty \dfrac {\paren {-1}^k} {1 + 2 k}\) | \(=\) | \(\ds \map \psi {\dfrac 3 4} - \map \psi {\dfrac 5 4}\) | multiplying both sides by $-1$ | ||||||||||
\(\ds \) | \(=\) | \(\ds \paren {-\gamma - 3 \ln 2 + \dfrac \pi 2} - \paren {-\gamma - 3 \ln 2 - \dfrac \pi 2 + 4}\) | Digamma Function of Three Fourths and Digamma Function of Five Fourths | |||||||||||
\(\ds \) | \(=\) | \(\ds \pi - 4\) | grouping terms | |||||||||||
\(\ds \leadsto \ \ \) | \(\ds 4 + 4 \sum_{k \mathop = 1}^\infty \dfrac {\paren {-1}^k} {1 + 2 k}\) | \(=\) | \(\ds \pi\) | adding $4$ to both sides | ||||||||||
\(\ds \leadsto \ \ \) | \(\ds 4 \times \paren {1 - \frac 1 3 + \frac 1 5 - \frac 1 7 + \cdots}\) | \(=\) | \(\ds \pi\) | |||||||||||
\(\ds \leadsto \ \ \) | \(\ds 4 \sum_{k \mathop = 0}^\infty \dfrac {\paren {-1}^k} {1 + 2 k}\) | \(=\) | \(\ds \pi\) |
$\blacksquare$
Also known as
Some sources refer to Leibniz's formula for $\pi$ as Gregory's series for $\pi$, for James Gregory.
Also see
Source of Name
This entry was named for Gottfried Wilhelm von Leibniz.
Historical Note
Leibniz discovered his formula for $\pi$ in $1673$.
He took great pleasure and pride in this discovery.
- It's as if, by this expansion, the veil which hung over that strange number had been drawn aside.
Simple as it is, Leibniz's Formula for Pi is inefficient, in that it needs hundreds of terms in order to calculate a few decimal places.
Some sources ascribe this formula to James Gregory.
Sources
- 1937: Eric Temple Bell: Men of Mathematics ... (previous) ... (next): Chapter $\text{VII}$: Master of All Trades
- 1968: Murray R. Spiegel: Mathematical Handbook of Formulas and Tables ... (previous) ... (next): $\S 19$: Series involving Reciprocals of Powers of Positive Integers: $19.15$
- 1986: David Wells: Curious and Interesting Numbers ... (previous) ... (next): $3 \cdotp 14159 \, 26535 \, 89793 \, 23846 \, 26433 \, 83279 \, 50288 \, 41972 \ldots$
- 1997: David Wells: Curious and Interesting Numbers (2nd ed.) ... (previous) ... (next): $0 \cdotp 78539 \, 81 \ldots$
- 1997: David Wells: Curious and Interesting Numbers (2nd ed.) ... (previous) ... (next): $3 \cdotp 14159 \, 26535 \, 89793 \, 23846 \, 26433 \, 83279 \, 50288 \, 41971 \ldots$
- 2009: Murray R. Spiegel, Seymour Lipschutz and John Liu: Mathematical Handbook of Formulas and Tables (3rd ed.) ... (previous) ... (next): $\S 21$: Series of Constants: Series Involving Reciprocals of Powers of Positive Integers: $21.15.$