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Dyfyniad Arddull ChicagoHeertje, Arnold. Economics and Technical Change. London: Weidenfeld & Nicolson, 1977.
Dyfyniad MLAHeertje, Arnold. Economics and Technical Change. London: Weidenfeld & Nicolson, 1977.
Rhybudd: Mae'n bosib nad yw'r dyfyniadau hyn bob amser yn 100% cywir.