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DTSTART;TZID=America/Toronto:20260305T143000
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URL:https://uwaterloo.ca/combinatorics-and-optimization/events/algebraic-en
 umerative-combinatorics-maria-gillespie-positive
SUMMARY:Algebraic & Enumerative Combinatorics - Maria Gillespie-A Positive\
 nCombinatorial Rule for Psi Class Products on Multicolored Spaces
CLASS:PUBLIC
DESCRIPTION:SPEAKER:\n Maria Gillespie\n\nAFFILIATION:\n Colorado State Uni
 versity\n\nLOCATION:\n MC 5417\n\nABSTRACT: We use a new combinatorial co
 nstruction and a\nsign-reversing involution to simplify an alternating sum
  that arises\nnaturally in intersection theory on moduli spaces of curves.
  In\nparticular\, it is well known that a product of ψ classes on the\nmo
 duli space M₀\,ₙ-bar\, the most commonly studied compactification\nof 
 the moduli space M₀\,ₙ of choices of n distinct marked points on\nℙ
 ¹\, is equal to a multinomial coefficient and has many natural\ncombinato
 rial interpretations.\n\nThere are similar ψ class products on other comp
 actifications of\nM₀\,ₙ\, including the \"multicolored\" spaces\, in w
 hich the answer is\na positive integer and yet only signed summation formu
 las were known.\nWe simplify the alternating sum formula in the multicolor
 ed case to\ngive a positive combinatorial rule\, and discuss some applicat
 ions of\nthe formula. This is joint work with Vance Blankers and Jake Levi
 nson.\nThere will be a pre-seminar presenting relevant background at the\n
 beginning graduate level starting at 1:30pm in MC 5417.
DTSTAMP:20260413T122145Z
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