A025724 Index of 7^n within sequence of numbers of form 6^i*7^j.

1, 3, 6, 10, 15, 21, 28, 36, 45, 55, 66, 78, 92, 107, 123, 140, 158, 177, 197, 218, 240, 263, 287, 312, 339, 367, 396, 426, 457, 489, 522, 556, 591, 627, 664, 703, 743, 784, 826, 869, 913, 958, 1004, 1051, 1099, 1148, 1198, 1250, 1303, 1357, 1412, 1468, 1525, 1583