#JavaScript (ES6), 42 bytes A port of [xnor's solution][1]. 0-indexed. <!-- language-all: lang-javascript --> f=(n,k=2)=>n<3||k<n&&f(k)*f(n+~++k)+f(n,k) [Try it online!](https://tio.run/##FctBDoIwEEDRPaeYBSEzDhqDS1rOQoPUaMkMAeOm1KtXu/qb91/u4/Zpe67vs@h9ztlblDbYjuwg5nYcwUjTeAx08ij8ZQ7EvhDKXjcUsHDtQcBAV8pMECuASWXXZb4s@sDRYR0l0Z/WscyURuqrlH8 "JavaScript (Node.js) – Try It Online") ---------- #JavaScript (ES6), <s> 83 </s> 75 bytes A faster, less recursive, but significantly longer solution. 0-indexed. <!-- language-all: lang-javascript --> f=(n,i,a=[p=1])=>a[n]||f(n,-~i,[...a,p+=(h=k=>k<i&&a[k]*a[i-++k]+h(k))(2)]) [Try it online!](https://tio.run/##FY1BCoMwEADvfcUeRLJNDK1XXT8SAi5WaxpJREsvar@e2tPAMDAv/vDaLW5@FyE@@pQGEkE5xWRmulukhk2w@z6ctvg6ZbTWrGZJYiRPja9dnrPx9srGFVJ6K0fhEUWJFtMQFxGA4FZBgBrKP6VE2C4AXQxrnHo9xadoWWRbOPBMs@084dFidTnSDw "JavaScript (Node.js) – Try It Online") [1]: https://codegolf.stackexchange.com/a/193186/58563