An even number 
for which 
.
The first few positive doubly even numbers are 4, 8,
12, 16, ... (OEIS A008586).
Doubly Even Number
See also
Even Function, Odd Number, Singly Even NumberExplore with Wolfram|Alpha
References
Sloane, N. J. A. Sequence A008586 in "The On-Line Encyclopedia of Integer Sequences."Referenced on Wolfram|Alpha
Doubly Even NumberCite this as:
Weisstein, Eric W. "Doubly Even Number." From MathWorld--A Wolfram Web Resource. https://mathworld.wolfram.com/DoublyEvenNumber.html