A075608 a(1) = 1, a(n) = smallest composite number not occurring earlier such that the concatenation a(n-1) and a(n) is a prime.

1, 9, 77, 27, 49, 33, 91, 51, 133, 39, 119, 69, 143, 21, 121, 57, 203, 93, 169, 63, 247, 81, 299, 147, 209, 123, 217, 87, 187, 111, 253, 153, 259, 99, 289, 129, 221, 159, 161, 141, 301, 177, 319, 117, 329, 201, 287, 219, 361, 183, 343, 237, 581, 171, 341, 273, 323