Pentagonal number theorem

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In mathematics, Euler's pentagonal number theorem relates the product and series representations of the Euler function. It states that ∏ n = 1 ∞ ( 1 − x n ) = ∑ k = − ∞ ∞ ( − 1 ) k x k ( 3 k − 1 ) / 2 = 1 + ∑ k = 1 ∞ ( − 1 ) k ( x k ( 3 k + 1 ) / 2 + x k ( 3 k − 1 ) / 2 ) . {\displaystyle \prod _{n=1}^{\infty }\left(1-x^{n}\right)=\sum _{k=-\infty }^{\infty }\left(-1\right)^{k}x^{k\left(3k-1\right)/2}=1+\sum _{k=1}^{\infty }(-1)^{k}\left(x^{k(3k+1)/2}+x^{k(3k-1)/2}\right).} In other words, ( 1 − x ) ( 1 − x 2 ) ( 1 − x 3 ) ⋯ = 1 − x − x 2 + x 5 + x 7 − x 12 − x 15 + x 22 + x 26 − ⋯ . {\displaystyle (1-x)(1-x^{2})(1-x^{3})\cdots =1-x-x^{2}+x^{5}+x^{7}-x^{12}-x^{15}+x^{22}+x^{26}-\cdots .} The exponents 1, 2, 5, 7, 12, ... on the right hand side are given by the formula gk = k(3k − 1)/2 for k = 1, −1, 2, −2, 3, ... and are called (generalized) pentagonal numbers (sequence A001318 in the OEIS). (The constant term 1 corresponds to k = 0 {\displaystyle k=0} .) This holds as an identity of convergent power series for | x | < 1 {\displaystyle |x|<1} , and also as an identity of formal power series. A striking feature of this formula is the amount of cancellation in the expansion of the product.