The sum of the digits should be a multiple of a perfect square number. # 4 is a perfect square number and a multiple of 4 + 3 + 1 = 8.
In the basic tests, 431 is included as one of the possible special primes, even though it does not meet the condition of its digit sum being a perfect square.
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In the basic tests, 431 is included as one of the possible special primes, even though it does not meet the condition of its digit sum being a perfect square.