This number is a prime.
Just showing those entries submitted by 'Haga': (Click here to show all)
The prime 623681 marks the greatest lead of 1 mod 4 primes over 3 mod 4 primes (counts 25444 vs. 25436) in the second region of negative values. Verified by actual count and documented in Hans Riesel, Prime numbers and computer methods for factorization, 2nd Edition (Boston: Birkhauser, 1994), p. 76. [Haga]