a(1) = 419 because there is only one 3 X 3 square arrangement of the primes 419, 421, 431, 433, 439, 443, 449, 457 and 461 such that all row, column and diagonal totals are primes. Here is the unique solution for the starting prime 419:
......++++
.......421..439..431.. > .1291
......++++
.......449..419..433.. > .1301
......++++
.......457..461..443.. > .1361
......++++
...../..vvv...vvv...vvv..\
..1301..1327..1319..1307..1283
