A number
is practical if for all , is the sum of distinct proper divisors of . Defined in 1948 by A. K. Srinivasen. All even perfect numbers are practical. The number

is practical for all ,
3, .... The first few practical numbers are 1, 2, 4, 6, 8, 12, 16, 18, 20, 24, 28,
30, 32, 36, 40, 42, 48, 54, 56, ... (OEIS A005153).
G. Melfi has computed twins, triplets, and 5-tuples of practical numbers. The
first few 5-tuples are 12, 18, 30, 198, 306, 462, 1482, 2550, 4422, ....