45

This number is a composite.

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The product of first 45 odd numbers minus 2, i.e., 1*3*5*7*…*89-2, is the largest prime less than a googol of this form. [Loungrides]

(45, 54) is the only pair of double-digit composite numbers (ab, ba) such that ab * ba + 1 is a 2-sphenic number, i.e., 45 * 54 + 1 = 2431 = 11 x 13 x 17. [Loungrides]