3867632931 × 10^10001 +1 - Numberphile

TL;DR

An enthusiast named Stefan discovered the world's largest "reversible prime" — a prime whose digits remain prime when reversed.

Key Points

• 1.An EMURP (prime spelled backwards) is a prime number whose digit-reversal is also prime — e.g., 1021 forward and 1201 backward are both prime.
• 2.The new record prime is 3,867,632,931 × 10^10,001 + 1: 10 digits, then 10,000 zeros, then a 1 — making it 10,011 digits total, beating the previous record by just 4 digits.
• 3.Stefan, a hobbyist, found it using a self-written "terrible" C prime sieve combined with established prime-testing algorithms from the PrimeGrid collaborative computing project.
• 4.The computation ran on a standard 8-core mini PC — the same machine shown in the video — making this an accessible amateur discovery.
• 5.Whether infinitely many EMURP primes exist is an open problem — this could theoretically be the last one ever found, which nobody has proven either way.