How to determine equation for $\sum_{k=1}^n k^3$

Question

How do you find an algebraic formula for $\sum_{k=1}^n k^3$? I am able to find one for $\sum_{k=1}^n k^2$, but not $k^3$. Any hints would be appreciated.

Answer 1

You can derive it from the formulas for the sums of lower powers:

$$\begin{align*} \sum_{k=1}^nk^3&=\sum_{k=1}^n\left(k\cdot k^2\right)\\ &=\sum_{k=1}^nk^2\sum_{i=1}^k1\\ &=\sum_{k=1}^n\sum_{i=1}^kk^2\\ &=\sum_{i=1}^n\sum_{k=i}^nk^2\\ &=\sum_{i=1}^n\left(\sum_{k=1}^nk^2-\sum_{k=1}^{i-1}k^2\right)\\ &=\sum_{i=1}^n\left(\frac16n(n+1)(2n+1)-\frac16i(i-1)(2i-3)\right)\\ &=\frac16n^2(n+1)(2n+1)-\frac16\sum_{i=1}^ni(i-1)(2i-3)\\ &=\frac16n^2(n+1)(2n+1)-\frac16\sum_{i=1}^n\left(2i^3-5i^2+3i\right)\\ &=\frac16n^2(n+1)(2n+1)+\frac56\sum_{i=1}^ni^2-\frac12\sum_{i=1}^ni-\frac13\sum_{i=1}^ni^3\\ &=\frac16n^2(n+1)(2n+1)+\frac5{36}n(n+1)(2n+1)-\frac14n(n+1)-\frac13\sum_{k=1}^nk^3\;, \end{align*}$$

and from here you can clearly solve for $\sum_{k=1}^nk^3$. Evidently this technique can be applied repeatedly, though it rapidly becomes very tedious.

Graham, Knuth, & Patashnik, Concrete Mathematics, offer many other approaches, including via finite calculus and generating functions.

Added (because I like it): The finite calculus approach requires first rewriting $k^3$ in terms of the falling factorials $k^{\underline1}=k$, $k^{\underline2}=k(k-1)=k^2-k$, and $k^{\underline3}=k(k-1)(k-2)=k^3-3k^2+2k$:

$$k^3=k^{\underline3}+3k^2+2k=k^{\underline3}+3k^{\underline2}+k^{\underline1}\;.$$

The coefficients are Stirling numbers of the second kind: in general $$x^n=\sum_{k=0}^n{n\brace k}x^{\underline k}\;.$$

Then

$$\begin{align*} \sum_{k=1}^nk^3&=\sum_{k=1}^n\left(k^{\underline3}+3k^{\underline2}+k^{\underline1}\right)\\ &=\left[\frac14k^{\underline4}+k^{\underline3}+\frac12k^{\underline2}\right]_1^{n+1}\\ &=\frac14n(n+1)(n-1)(n-2)+n(n+1)(n-1)+\frac12n(n+1)-0\\ &=\frac14n(n+1)\Big(n^2-3n+2+4(n-1)+2\Big)\\ &=\frac14n(n+1)\left(n^2+n\right)\\ &=\frac14n^2(n+1)^2\;. \end{align*}$$

There is a nice introduction to finite calculus in Section $2.6$ of Graham, Knuth, & Patashnik, Concrete Mathematics; Pete L. Clark has a handout with another introduction here.

Answer 2

The sum is $\dfrac{n^2(n+1)^2}{4}$.

The formula can be proved correct by induction. But there is also a nice geometric proof that can be found, for example, in the book Proofs Without Words.

For a general discussion of related problems, please see this Wikipedia article on Faulhaber's Formula.

Remark: There are various ways to discover the formula. We can calculate the sum of the first $n$ cubes for various small $n$. The actual formula is so simple that the answer leaps out. Then we can verify that itis correct by checking that $$\frac{(k^2)(k+1)^2}{4}-\frac{(k-1)^2(k^2)}{4}=k^3.$$ In this case, the calculation is instantaneous, for we are looking at a difference of squares.

Or else suppose we already know formulas for $\sum_1^n k$ and $\sum_1^n k^2$. Observe that $$(k+1)^4-k^4=4k^3+6k^2+4k+1.$$ Sum from $k=1$ to $k=n$. On the right we get a lot of cancellation, and we get $$(n+1)^4-1^4=4\sum_1^n k^3 +6\sum_1^n k^2+4\sum_1^n k +\sum_1^n 1.$$ We know everything in the equation above except $\sum_1^n k^3$. So now we know that too.

Answer 3

As you probably have already realized the formula $$ \sum_{k=1}^n k^3 = \left(\frac{n(n+1)}{2}\right)^2 $$ can be proved using induction.

For $n=1$ you have $$ 1^3 = \left(\frac{1\cdot 2}{2}\right)^2. $$

Say that the formula holds for $n$ and prove that it holds for $n=1$. So you have/get $$ \sum_{k=1}^{n+1} k^3 = \left[\sum_{k=1}^n k^3\right] + (n+1)^3 = \frac{n^2(n+1)^2}{2^2} + (n+1)^3. $$ I will leave you to prove that this right hand side is indeed equal to $$ \frac{n^2(n+1)^2}{2^2} + (n+1)^3 = \dots = \frac{(n+1)^2(n+2)^2}{2^2}. $$

Answer 4

The general approach is to write $p(k)$ in terms of the polynomals $\binom{k}{i}$ with $i=0,1,2,\dots$

For example, $$k^3 = 6\binom k 3 + 6\binom k 2 + \binom k 1$$

Now, since $$\sum_{k=1}^{n} \binom{k}{i} = \binom{n+1}{i+1}$$

you get:

$$\sum_{k=1}^{n} k^3 =6\binom{n+1}{4} + 6\binom{n+1}{3} + \binom{n+1}{2}$$

(There is a slight problem above when $i=0$. You really need sums from $k=0$ to $n$ for that case. In all other cases, $k=0$ doesn't add anything.)

Answer 5

In addition to Brian M. Scott's answer, and the reference to Concrete Mathematics, you can actually derive this identity using higher powers and perturbation method

Keep in mind that $$ \sum_{k=1}^{n}k=\frac{n(n+1)}{2}\\ \sum_{k=1}^{n}k^2 = \frac{n(n+1)(2n+1)}{6} $$ So you start with $$ S_n=\sum_{k=1}^{n}k^4\\ S_{n+1}=S_n +(n+1)^4 = \sum_{k=1}^{n}(k+1)^4+1 =S_n+4 \sum_{k=1}^{n}k^3+6 \sum_{k=1}^{n}k^2 +4 \sum_{k=1}^{n}k +n+1 $$ hence $$ S_n +(n+1)^4=S_n+4 \sum_{k=1}^{n}k^3+6 \sum_{k=1}^{n}k^2 +4 \sum_{k=1}^{n}k +n+1 $$ So clearly the highest term cancels out and after some algebra you will get your $$ \sum_{k=1}^{n}k^3=\frac{(n(n+1))^2}{4} $$

Answer 6

Here is another approach,

$$ \sum_{k=1}^{n} (k+1)^4 - \sum_{k=1}^{n} k^4 = (n+1)^4-1 $$

$$ \implies 4\sum_{k=1}^{n}k^3 + 6\sum_{k=1}^{n} k^2 + 4\sum_{k=1}^{n}k + \sum_{k=1}^{n}1 = (n+1)^4 -1 $$

$$ \implies 4\sum_{k=1}^{n}k^3 = (n+1)^4-1-6\sum_{k=1}^{n}k^2 - 4\sum_{k=1}^{n}k - \sum_{k=1}^{n}1$$

$$ \implies \sum_{k=1}^{n}k^3 = \dots. $$

Answer 7

One classic method is

$$\begin{align} \sum_{k=1}^{n} k^4 &= \left( \sum_{k=1}^n (k+1)^4 \right) + 1 - (n+1)^4 \\&= \left( \sum_{k=1}^n k^4 + 4 k^3 + 6 k^2 + 4 k + 1 \right) + 1 - (n+1)^4 \\&= \left( \sum_{k=1}^n k^4 \right) + 4 \left( \sum_{k=1}^n k^3 \right) + 6 \left( \sum_{k=1}^n k^2 \right) + 4 \left( \sum_{k=1}^n k^1 \right) + \left( \sum_{k=1}^n 1 \right) + 1 - (n+1)^4 \end{align}$$

The sum of fourth powers cancel out, and you can solve the equation for the sum of curves.

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Another approach is to guess that the sum of cubes should be a fourth degree polynomial, then solve for the coefficients of the polynomial using the equations

• $f(0) = 0$
• $f(1) = 1$
• $f(2) = 1 + 2^3$
• $f(3) = 1 + 2^3 + 3^3$
• $f(4) = 1 + 2^3 + 3^3 + 4^3$

Five equations in five unknowns. You can cut this down to 2 unknowns if you recognize some easy patterns: the leading coefficient for the sum of $n$-th powers is always $1/n$ and the next is always $1/2$, and the constant term is always 0.

Answer 8

$$ \sum_{k=1}^nk^3=\bigg(\sum_{k=1}^n k\bigg)^2. $$ Can you get the intuition from the following two pictures?

The images are from Brian R Sears (Wayback Machine).