A078271 - OEIS

A078271

a(n) is the number from which if even-positioned digits are deleted then one gets the concatenation of 1 to n, while if odd-positioned digits are deleted then one gets reverse concatenation of 1 to n. Position of least significant digit is considered to be 1.

11, 2112, 312213, 41322314, 5142332415, 615243342516, 71625344352617, 8172635445362718, 918273645546372819, 1102938475665748392110, 11121304958677685941302111, 112213141506978879615041312112