121661

This number is a prime.

+ The 121st prime is 661. If we concatenate 121 and 661, or 661 and 121, we get two more primes (121661 and 661121). This is the smallest case of what high school algebra teacher Terry Trotter (1941-2004) called OP-PO primes (O = Ordinal position, and P = Prime).

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