Why is in OSPF LSA sequence number in range 0x80000001 to 0x7FFFFFFF. I suppose that it is for some historical reasons but cannot google it.
LSA sequence number is a signed 32-bit integer.
So 0x80000001 to 0x7fffffff basically covers the whole possible range except one number: 0x80000000, and this special number is reserved.
You can read this page of RFC 2328 for more details.