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<div align="center"><big><b><big><br>
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<div align="center"><big><b><big>Welcome to the 2021
Fall talks of ODTU-Bilkent Algebraic
Geometry Seminars</big></b></big><b><br>
</b><b> </b><b> </b></div>
<b> </b>
<div align="center"><i>since 2000</i><br>
<b> </b> </div>
<div align="center"><b> </b><b> </b><b><b>=================================================================</b>
</b><br>
<b> </b><br>
This week the <a
href="http://www.bilkent.edu.tr/%7Esertoz/agseminar.htm"
target="_blank">ODTU-Bilkent Algebraic Geometry
Seminar</a> is <b>Online.</b> <br>
<br>
<i><font color="#ff00ff">This talk will begin at <u><b>15:40</b></u><u>
(GMT+3)</u></font></i><br>
<br>
<b>=================================================================</b></div>
<div align="center"><br>
<br>
<div align="center"><b><img
src="cid:part1.8NLG0pLX.dLIPEAvR@bilkent.edu.tr"
alt="image.png" width="410" height="516"> </b><font
size="-1"><i> </i></font></div>
<div align="center"><font size="-1"><i>Paul
Gauguin (1848-1903)<br>
</i></font></div>
<div align="left"><br>
<b><b><font color="#ff0000">Speaker:<font
color="#000000"> Sergey Finashin<br>
</font></font></b></b></div>
<div align="left"><b><b><font color="#ff0000">Affiliation:
<font color="#000000">ODTÜ</font><br>
Title:<font color="#000000"> Two kinds of
real lines on real del Pezzo surfaces
and invariance of their signed count<br>
</font><br>
</font></b></b></div>
</div>
<div align="left"><b><b><font color="#ff0000">Abstract:</font></b></b><font
color="#ff0000"><font color="#000000"> In his
classical treatise on real cubic surfaces,
Segre discovered two kinds of real lines which
he called elliptic and hyperbolic.<br>
<br>
His enumeration indicated that the number of
hyperbolic is greater by 3 than the number of
elliptic ones independently of a real
structure on the cubic surface.<br>
<br>
However this property did not receive a
conceptual explanation until recently: in a
joint work with V.Kharlamov we interpreted a
signed count of lines as a signed count of
zeroes of some vector field in a Grassmannian
(and so, it is Euler’s number of the
corresponding vector bundle).<br>
<br>
In the current work that I will present, we
develop an alternative approach to counting
lines on real del Pezzo surfaces X of degrees
1 and 2 (a projective plane blown up at 8 or 7
generic points, respectively). The two types
of real lines are distinguished by certain
canonical Pin-structure on the real locus X_R
(this looks different from the approach of
Segre, but is equivalent to it in the case of
cubic surfaces).<br>
<br>
A signed count of real lines is interpreted as
some lattice root enumeration, which lets us
prove our invariance properties for del Pezzo
of degree 1 and 2, like in the case of cubic
surfaces.<br>
</font></font></div>
<div align="left"><font color="#ff0000"><font
color="#000000"><br>
</font></font></div>
<div align="left"><font color="#ff0000"><font
color="#000000"><b><font color="#ff0000">Date:<font
color="#000000"> 5 November 2021</font></font></b>,
Friday</font></font><br>
</div>
<div align="left"><font color="#ff0000"><font
color="#000000"><b> <font color="#ff0000">Time:
</font>15:40 (GMT+3)</b><br>
<b><font color="#ff0000">Place: </font></b><font
color="#ff0000"><font color="#000000"><b>Zoom<br>
</b></font></font></font></font></div>
<div align="left"><font color="#ff0000"><font
color="#000000"><font color="#ff0000"><font
color="#000000"><b><br>
</b></font></font></font></font></div>
<blockquote>
<p><font color="#ff0000"><font color="#000000"><font
color="#ff0000"><font color="#000000"><b><b
style="color:rgb(51,51,51);font-family:arial,verdana,sans-serif;font-size:14px">One
day before the seminar, an
announcement with the Zoom meeting
link will be sent to those who
registered with Sertöz. <br>
</b></b></font></font></font></font></p>
<p><font color="#ff0000"><font color="#000000"><font
color="#ff0000"><font color="#000000"><b><b
style="color:rgb(51,51,51);font-family:arial,verdana,sans-serif;font-size:14px">If
you have registered before for one
of the previous talks, there is no
need to register again; you will
automatically receive a link for
this talk too.<br>
</b></b></font></font></font></font></p>
<p><font color="#ff0000"><font color="#000000"><font
color="#ff0000"><font color="#000000"><b><b
style="color:rgb(51,51,51);font-family:arial,verdana,sans-serif;font-size:14px">If
you have not registered before,
please contact him at <a
href="mailto:sertoz@bilkent.edu.tr?subject=Zoom%20Seminar%20Address%20Request"
target="_blank">sertoz@bilkent.edu.tr</a>.</b></b></font></font></font></font></p>
</blockquote>
<p><font color="#ff0000"><font color="#000000"><font
color="#ff0000"><font color="#000000"><b> </b></font></font></font></font></p>
<div align="left"><br>
</div>
<div align="left">Please bring your own tea and
cookies and self-serve at the convenience of your
own home! 😁<span></span></div>
<div align="left"><br>
</div>
<div align="left">You are most cordially invited to
attend.</div>
<div align="left"><br>
</div>
<div align="left">Ali Sinan Sertöz<br>
<font color="#ff0000"><font color="#000000"><b><font
color="#ff0000"> </font></b></font></font></div>
<pre cols="72"><div style="text-align:left"><span style="font-family:Arial,Helvetica,sans-serif">------------------------------</span><span style="font-family:Arial,Helvetica,sans-serif">------------------------------</span><span style="font-family:Arial,Helvetica,sans-serif">---------------- </span></div><div style="text-align:left"><span style="font-family:Arial,Helvetica,sans-serif">Ali Sinan Sertöz</span></div><div style="text-align:left"><span style="font-family:Arial,Helvetica,sans-serif">Bilkent University, Department of Mathematics, 06800 Ankara, Turkey</span></div><div style="text-align:left"><span style="font-family:Arial,Helvetica,sans-serif">Office: (90)-(312) - 290 1490</span></div><div style="text-align:left"><span style="font-family:Arial,Helvetica,sans-serif">Department: (90)-(312) - 266 4377</span></div><div style="text-align:left"><span style="font-family:Arial,Helvetica,sans-serif">Fax: (90)-(312) - 290 1797</span></div><div style="text-align:left"><span style="font-family:Arial,Helvetica,sans-serif">e-mail: </span><a href="mailto:sertoz@bilkent.edu.tr" style="font-family:Arial,Helvetica,sans-serif" target="_blank" class="moz-txt-link-freetext">sertoz@bilkent.edu.tr</a><span style="font-family:Arial,Helvetica,sans-serif"> </span></div><div style="text-align:left"><span style="font-family:Arial,Helvetica,sans-serif">Web: </span><a href="http://sertoz.bilkent.edu.tr" style="font-family:Arial,Helvetica,sans-serif" target="_blank">sertoz.bilkent.edu.tr</a><span style="font-family:Arial,Helvetica,sans-serif"> </span></div><div style="text-align:left"><span style="font-family:Arial,Helvetica,sans-serif">------------------------------</span><span style="font-family:Arial,Helvetica,sans-serif">------------------------------</span><span style="font-family:Arial,Helvetica,sans-serif">----------------</span></div></pre>
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