A figurate number constructed by taking the (3n - 2)th tetrahedral number and removing the (n - 1)th tetrahedral number from each of the four corners, Ttet_n | congruent | Te_(3n - 3) - 4Te_(n - 1) | = | 1/6 n(23n^2 - 27n + 10). The first few are 1, 16, 68, 180, 375, ... (OEIS A005906). The generating function for the truncated tetrahedral numbers is (x(10x^2 + 12x + 1))/(x - 1)^4 = x + 16x^2 + 68x^3 + 180x^4 + ....