Fall 2014

Equations for the Fifth Secant Variety of Segre Products of Projective Spaces

Tuesday, Dec. 15, 2015 10:40 am11:15 am PST

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Calvin Lab

It was known classically that $2 \times 2 \times 2 \times 2 $ tensors are defective in rank 3 -- the generic tensor of that format and rank has infinitely many decompositions. 
Catalisano, Geramita and Gimigliano showed that binary tensors for $5$ or more factors are not defective, so tensors of rank less than the generic rank have finitely many decompositions. 
Bocci and Chiantini showed that $2 \times 2 \times 2 \times 2 \times 2$ tensors are not identifiable in rank 5 ---  the generic tensor of that format has exactly 2 decompositions up to symmetry.   
Bocci, Chiantini and Ottaviani showed that for $n \geq 6$ factors, binary $n$-factor tensors are almost always $k$-identifiable.
Bernd Sturmfels asked in his ``algebraic fitness session'' at the Simon's Institute (Fall 2014) to study the boundary case of binary 5 factor tensors of rank 5 and to find its minimal defining equations. 
In this talk I will describe a computational proof that the fifth secant variety of the Segre product of five copies of the projective line is a codimension 2 complete intersection of equations of degree 6 and 16. Our computations rely on pseudo-randomness, and numerical accuracy, so parts of our proof are only valid “with high probability”.
This is joint work with Steven Sam.