Leffler, G. L. (1952). Seasonal changes in Pennsylvania business.
शिकागो शैली (17वां संस्करण) प्रशस्ति पत्रLeffler, George Leland. Seasonal Changes in Pennsylvania Business. State College, 1952.
एमएलए (8वां संस्करण) प्रशस्ति पत्रLeffler, George Leland. Seasonal Changes in Pennsylvania Business. 1952.
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