Hodgetts, Richard M.
The Business enterprise : social challenge, social response
Saunders, 1977
 Buku Teks SO
Wisnu Murdiono
Protokol identifikasi challenge-response dengan implementasinya berupa passcode generator pada platform J2ME dan bluetooth
Fakultas Ilmu Komputer Universitas Indonesia, 2004
 UI - Skripsi Membership
Kouzes, James M.
The leadership challenge
Jossey-Bass, 2002
 Buku Teks SO
Myrdal, Gunnar, 1898-
Challenge to affluence
Pantheon Books, 1963
 Buku Teks
Miner, John B.
The challenge of managing
W.B. Saunders, 1975
 Buku Teks