661

This number is a prime.

+ 661 is the start of a record-breaking twin prime gap.

+ Lord Kelvin, one of Britain's great physicists, published 661 papers on a wide range of scientific subjects. [Rosen]

+ The smallest prime that becomes reversible (emirp) when turned upside-down (199). [Beedassy]

+ The smallest prime p that divides the sum of cubes of the first p primes. [Beedassy]

+ 661 (the 11th star number) is the 121st prime (121 is the 5th star number). In the Hebrew Bible, Esther (אסתר), which means "star" (Strongs H635), has a gematria value of 661. [Slattery]

(There are 4 curios for this number that have not yet been approved by an editor.)

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