900
This number is a composite.
The smallest square which is the sum of distinct primes using all ten digits (900 = 503+241+89+67 = 509+283+61+47). [Capelle]
The sum of square of first 900 prime numbers yields another prime {13459558559}. Note that 900 is also a square number. [Bajpai]
(There is one curio for this number that has not yet been approved by an editor.)
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