The reference is at the bottom of this, author probably forgot to fix it when cutting and pasting from Mathworld (yes, it's the same Gardner) https://mathworld

The reference is at the bottom of this, author probably forgot to fix it when cutting and pasting from Mathworld (yes, it's the same Gardner) https://mathworld.wolfram.com/BettiNumber.html https://mathworld.wolfram.com/BettiNumber.html