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Mathematics
Mathematics
Problem
Let the set of all relations
R
R
R
on the set
{
a
,
b
,
c
,
d
,
e
,
f
}
\{a, b, c, d, e, f\}
{
a
,
b
,
c
,
d
,
e
,
f
}
, such that
R
R
R
is reflexive and symmetric, and
R
R
R
contains exactly 10 elements, be denoted by
S
\mathcal{S}
S
. Then the number of elements in
S
\mathcal{S}
S
is ____.
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