a(n) = {
	if(n <= 2, return(11-n));
	if(n%2 == 1,
		return(9*10^(n-1));
	); 
	my(res = 0, nover2 = n/2, nover2p1 = n/2 + 1, nover2m1 = n/2 - 1);
	forvec(x = vector(n-1, i, [0, 9]),
		if(x[nover2m1] == x[nover2],
			res-=qperms(x);
			res+=qperms(concat(0,x));
		)
	,
	1
	);
	forvec(x = vector(n, i, [1, 9]),
		if(x[nover2] == x[nover2p1],
			res+=qperms(x);
		);
	,
	1
	);
	res
}

addhelp(qperms, "number of permutations elements in vector v.")
qperms(v) = {my(r=1,t); for(i=1,#v-1,if(v[i]==v[i+1],t++,r*=binomial(i,t+1);t=0));r*=binomial(#v,t+1)}

/*
from qperms I removed a v = vecsort(v) since input comes in sorted from function a
*/