Neither for a nor for b are we able to decide whether it is 0 or not. But ab=0.

http://books.google.co.jp/books?id=qfp_-Fo9yWMC&;pg=PA24&lpg=PA24&dq=ab%3D0+Intuitionism&source=bl&ots=AmcPPZa-e8&sig=VMxI0HptfZHbmyvI2m_LAkHAm50&hl
=ja&sa=X&ei=ucGKU5LDFYSRkgXhm4GoDg&ved=0CCkQ6AEwAA#v=onepage&q=ab%3D0%20Intuitionism&f=false

URLが長いので途中改行