Wonderful Demlo numbers are the squares of successive repunits: 1²=1, 11²=121, 111²=12321, …, 111111111²=12345678987654321.
Indian mathematician D. R. Kaprekar studied and named them after Demlo railway station (now Dombivli), where he conceived the idea in 1923. He published the results in 1938.
The digit pattern holds exactly while n≤9 because multiplication produces no carries.