Which marketing strategy led to the significant growth of Springer's 'Yellow Series' of advanced mathematics textbooks in the 1980s and 1990s?

The mathematics textbook industry has evolved significantly over the decades, adapting various marketing strategies to appeal to different educational institutions and students. Publishers have used fascinating approaches to differentiate their products in this competitive market. This poll tests your knowledge about a groundbreaking marketing strategy that revolutionized how advanced mathematics textbooks were positioned in the academic marketplace. Can you identify the innovative approach that changed the game?

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Celebrity mathematician endorsements featuring Fields Medal winners
Consistent visual branding with yellow covers and standardized design that made the series instantly recognizable in libraries
Free online companion software with every textbook purchase
Buy-one-get-one promotions targeting university procurement departments