Delicious math: Cocktails inspired by 13th-century numbers wiz at Friday Saturday Sunday

Paul MacDonald says he was a terrible math student. And then he became a bartender who discovered he was in tune with a famous 13th-century numbers wiz. MacDonald, whose complex-yet-elegant cocktails have been a prime draw to Rittenhouse haunts like Friday Saturday Sunday (and a.Bar before that), was tinkering with a blend of fortified wines one day when he realized the proportions of his recipe mirrored the Fibonacci sequence, in which each number is found by adding up the two previous numbers, beginning with 0 and 1, then another 1, 2, 3, 5, 8, 13, etc. Fibonacci, also known as Leonardo of Pisa, was pondering in 1202 an equation to predict the prolific breeding habits of rabbits, but the sequence has since been used to describe spiraling things in nature, art, and music, including hurricanes, the head of a sunflower, and a Mozart sonata. As the cocktailian apostle of the theory, Paul of Rittenhouse has invented at least a dozen seasonal drinks based on the concept. And his current off-menu offering at FSS, Fibonacci in Springtime, is a perfect example of how remarkably harmonius a drink can be when it's balanced on the tip of a spiral. In rising quantities, MacDonald blends savory Leatherbee Fernet from Chicago with citrusy, light Amaro Montenegro, floral elderflower liqueur, dry red Dolin vermouth, and an unusual French gin, G-Vine Floraison, made of grape spirit infused with grape leaves. With a giant bobbing ice cube to chill it down, and a thick curl of grapefruit zest hanging off the tumbler's crystal lip, this drink unfurls with different notes on every sip -- a hint of caraway, a citrusy bitter smack, a wave of buttery richness, and the lingering floral ghost of vineyard. Never has advanced math been so delicious.

-- Craig LaBan

Fibonacci in Springtime, $14, Friday Saturday Sunday, 261 S. 21st St., 215-546-4232; fridaysaturdaysunday.com