[Solution] Clearly, all terms of the sequence are positive integers.
Since when n is even,
an+1=21an<an
Therefore, the smallest term of the sequence must be odd.
Let the smallest term of the sequence be ap, then ap is odd, and
ap+1=ap+7ap+2=21ap+1=21(ap+7)
By the minimality of ap, we have
ap⩽ap+2
That is,
ap⩽21(ap+7)ap⩽7
Noting that ap is odd. Therefore, ap has only 4 possible values:
1,3,5,7
Since
1994≡−1(mod3)
Therefore
19941995≡(−1)1995=−1≡2(mod3)
That is, there exists a positive integer t, such that
19941995=3t+2
Since 1994 is even, t is even, let t=2s, we get
19941995=6s+2
Here s is a positive integer.
Thus, we have
a1=199319941995=19936s+2=(19936)s⋅19932≡1s⋅(−2)2(mod7)≡4(mod7)
From a1≡4(mod7), we know that the remainders of the terms of this sequence when divided by 7 can only be 4,2,1. Therefore, ap cannot be 3,5,7, the value of the smallest term of the sequence
ap=1