India, Maharashtra, Mumbai (Bombay), Victoria Terminus railways station or Chhatrapati Shivaji Photograph: Tuul & Bruno Morandi/Getty Images
By day, Shyam Sunder Gupta was Principal Chief Engineer of Indian Railways. By night, he was a guru of recreational mathematics.
For decades, Gupta spent his free time exploring patterns in numbers, his numerical curiosities finding their way into journals, magazines and books.