Emirpimes

Define an emirpimes ("semiprime" spelled backwards) as a semiprime whose (base 10) reversal is a different semiprime. The first such number is 15, because 15 reversed is 51 and both 15 and 51 are semiprimes (i.e.,  and ). A list of the first emirpimeses (or "semirpimes") are 15, 26, 39, 49, 51, 58, 62, 85, 93, 94, 115, 122, 123, ... (OEIS A097393). A binary plot of the semirpimes is illustrated above.

The numbers of semirpimes less than  for , 2, ... are 0, 10, 90, 898, 7200, 60732, ... (OEIS A097394).

REFERENCES:

Sloane, N. J. A. Sequences A097393 and A097394 in "The On-Line Encyclopedia of Integer Sequences."

Referenced on Wolfram|Alpha: Emirpimes