Seven is written as a bar and two dots, and it stands at the centre of the thirteen, with six numbers below it and six above. Seven recurs in Maya narrative in a way the neighbouring numbers do not: the Popol Vuh names Seven Death among the lords of Xibalba, and Seven Macaw is the false sun the Hero Twins bring down.
Those are calendar names. One Death and Seven Death, One Hunahpu and Seven Hunahpu, carry numbers and day signs in exactly the form a Tzolk'in day takes, which is one of the clearest indications that the K'iche' narrative and the day count belong to a single system of thought rather than merely coexisting.
Maya numerals are written with a dot for one and a bar for five, and the coefficient of a day is written in exactly that way in the inscriptions, set beside the day sign in its cartouche. The system is positional and includes a sign for zero, which is why the Long Count can be written as a string of places, but within the 260-day count no number above thirteen ever occurs and the notation stays simple.
The term Resonant, used for this coefficient in contemporary popular literature, is from Jose Arguelles's Dreamspell of 1987. It is a modern coinage; the Popol Vuh names are Maya, and the two should not be run together, since the first is an invention of the twentieth century and the second is a sixteenth-century transcription of an older tradition.
Sources Maya Hieroglyphic Writing: An IntroductionJ. Eric S. Thompson with Dreamspell: The Journey of Timeship Earth 2013Jose Arguelles