An even number for which . The first few Positive doubly even numbers are 4, 8, 12, 16, ... (Sloane's A008586).
See also Even Function, Odd Number, Singly Even Number
References
Sloane, N. J. A. Sequence
A008586
in ``The On-Line Version of the Encyclopedia of Integer Sequences.''
http://www.research.att.com/~njas/sequences/eisonline.html.