What is the fastest way to multiply two integers?  (from the Prime Pages' list of frequently asked questions)
 New record prime: 277,232,917-1 with 23,249,425 digits by Pace, Woltman, Kurowski, Blosser & GIMPS (26 Dec 2017).
 Question: I would like to know what is currently the fastest algorthim used to multiply two arbitary long integers. I would also like to know if the function is available in C/C++. Yves Gallot replies: It depends on the size of the numbers: up to about 100 digits, the grammar-school method is the fastest between 100-1,000 digits, the Karatsuba method is the fastest (a recursive formula that replace 4 multiplications by 3). between 1,000-10,000,000 digits, convolution based on FFT using some floating-point numbers is the fastest for number having more than 10,000,000 digits, multiple Number Theoretic Transforms and Chinese Remainder Theorem should be used, because accuracy of floating-point numbers of available processors is not large enough.
 Another prime page by Chris K. Caldwell