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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 5 curios for this number that have not yet been approved by an editor.)
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