The Engel expansion of a positive real number x is the unique non-decreasing sequence of positive integers {\displaystyle } such that x = 1 a 1 + 1 a 1 a 2 + 1 a 1 a 2 a 3 + ⋯ = 1 a 1 {\displaystyle x={\frac {1}{a_{1}}}+{\frac {1}{a_{1}a_{2}}}+{\frac {1}{a_{1}a_{2}a_{3}}}+\cdots ={\frac {1}{a_{1}}}\! Wikipedia