Lemma. Suppose n≥2 is an integer, then all of the numbers (n0),(n1),…,(nn) are odd if and only if n=2k−1 for an integer k≥2.
*Proof*. For an integer t, let v(t) be the greatest integer u such that 2u∣n. We know that
" If p,q∈N and 0≤p≤2q then v(p)=v(2q+p)=v(2q−p)" (1)
Because if p=2ab with a,b∈N and b an odd number then a<q and 2q±p=2a(2q−a±b) where 2q−a±b are odd numbers.
Now there is one and only one m∈N such that 2m≤n<2m+1. Let n=2m+s
with 0≤s<2m. Now consider the number (n2m−1). We have
(n2m−1)=(2m+s2m−1)=(2m+ss+1)=s(s−1)⋯(1)(s+1)(2m+s)(2m+s−1)⋯(2m+1)(2m).
By (1) we have v(2m+i)=v(i) where 1≤i≤s, therefore
v((2m+s)⋯(2m+1))=v(s!),
and by assumption (n2m−1) is odd therefore v(2m)=v(s+1) and consequently
2m∣s+1 and s+1≥1. Hence we have 2m−1≤s and therefore s=2m−1 and
n=2m+s=2m+1−1.
Now if n=2k−1 for some natural number k, for each 1≤c≤n we have
(2k−1c)=(1)(2)⋯(c)(2k−1)(2k−2)⋯(2k−c)and by (1) we know for1≤l≤c
v(2k−l)=v(l), so (2k−1c) is odd for 0≤c≤n.□
Now we return to main problem:
Positive integer n has the property of the problem if and only if all the numbers (ni)−i (0≤i≤n) have the same parity. It means that for every 0≤i≤n−1, (ni),(ni+1) have different parities. So (n+1i+1)=(ni)+(ni+1) (1≤i≤n−1) is odd
and as we know (n+10)=1 is also odd. Therefore by the lemma this is equivalence
to n+1=2k−1 for some integer k≥2 so n=2k−2 where k≥2 is an integer.