Definition: In geometry, a septagonal number is a figurate number that can be represented as a 12-sided polygon. It's named after the septa, which are thirteen points on a regular hexagon, forming the sides of the polygon. A septagonal number is defined by the formula: \[ \text{septang} = n(n + 1) / 2 \] where n is the term we're considering. The first few septangular numbers are:
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