a(5) = 49:
+----+----+----+----+----+----+----+----+----+----+----+----+----+
|    |    |    |    |    |    |    |    |    | 42 |    | 25 | 49 |
+----+----+----+----+----+----+----+----+----+----+----+----+----+
|    |    |    |    | 45 |    |    |    | 41 |    |  1 | 24 |    |
+----+----+----+----+----+----+----+----+----+----+----+----+----+
|    |    |    |    | 29 | 16 | 31 |    | 27 | 14 |    | 23 | 47 |
+----+----+----+----+----+----+----+----+----+----+----+----+----+
|    |    |    | 12 |  1 |    | 15 |    | 13 |    | 22 |    |    |
+----+----+----+----+----+----+----+----+----+----+----+----+----+
|    |    | 44 |    | 11 | 10 | 43 |  5 | 38 |  8 |    | 40 |    |
+----+----+----+----+----+----+----+----+----+----+----+----+----+
|    |    |    | 32 | 21 |    |  9 |  4 |  1 |  7 | 18 |    |    |
+----+----+----+----+----+----+----+----+----+----+----+----+----+
| 37 |    | 33 |    |    | 30 |    | 17 |  3 |  2 |  1 | 19 | 39 |
+----+----+----+----+----+----+----+----+----+----+----+----+----+
| 36 |  1 | 34 |    |    |    |    | 48 | 28 |  6 |    | 20 |    |
+----+----+----+----+----+----+----+----+----+----+----+----+----+
|    | 35 |    |    |    |    |    |    |    |    | 26 | 46 |    |
+----+----+----+----+----+----+----+----+----+----+----+----+----+