In order to obtain the day of the week from the date, we can use

where *k* is the day of the month, *m* is the month ( , ...,
, , ),
*C* is century (1990 has *C*=19), *Y* is the year (1990 has *Y*=97
except *Y*=96 for Jan. and Feb.) *W* is the day of the week
( , ..., ).
(Jan. and Feb. are treated as months of the preceding year).
(see [&make_named_href('',
"node5.html#zwillinger","[1]")], page 738.)
In order to obtain savant's ability, the computation would better
be simple and convenient. At least it is required that

- The number of numbers kept by heart is at most ten to twenty.
- The computation does not contain more than two-figure calculations.
- The number of calculation steps is at most few steps.