** JRF
Position:** Those interested email me your curriculum vitae.

** About Me**

B. Tech.

College of engineering,
Thiruvananthapuram (CET)
M. Tech.

IIT,
Madras
Ph.D

Institute of
Mathematical Sciences, Chennai (with Prof.

Kamal Lodaya)

Other Research Positions:

University
of Tubingen,

TIFR - Mumbai,

LIAFA - Paris,

CMI - Chennai,

University of Warsaw - Poland
** PhD Thesis: ** Regular
quantifiers in Logic
(won

ACM
India Honourable Mention for the dissertation)

**Research**

Modulo counting in Logics (

click
for details)

Linear
temporal logic (LTL) and similarly first order logic (FO) over words
cannot do modulo counting. For example, the language (aa)*
is not definable in LTL. In our paper [Sre],
we showed the equivalence of modulo counting extensions of FO and LTL
(called FOMOD, LTLMOD respectively).
We also looked at the complexity of satisfiability of LTLMOD, various
sublogics [LS] and
two variable fragments of FO [LS].
Descriptive complexity (

click
for details)

First order logic (using arbitrary arithmetic predicates) over words is
expressively equivalent to AC^{0} circuits. Similarly,
modulo counting quantifiers and CC^{0} circuits (see this book
by Howard Straubing) are closely related.
The relation between many of these circuit classes are open. We look at
these questions from the perspective of logic.
In our paper [KS] we
show that these classes are different for "highly uniform" circuit
classes.
Linear Orderings (

click for details)

From the work of Schutzenberger we understand the relation between
first order logic and monadic second order logic over finite words. In
our work [CS]
we looked at the relation between many logics (including FO, WMSO etc)
over countable linear orderings.
In another work [MS], we gave
similar algebraic characterization for two variable FO.
Software Verification (

click for
details)

This is my current interest. The objective is to use logic and automata
in Software verification.

** Publications**

8.

Two-variable first
order logic with counting quantifiers: Complexity
Results, with

Kamal
Lodaya,

DLT 2017.
(

pdf,

abstract)

** Abstract **:
Etessami,
Vardi and Wilke showed that satisfiability of two-variable first order
logic FO^{2}[<] on word models is Nexptime-complete.
We extend this upper bound to the slightly stronger logic
FO^{2}[<,successor,equiv], which allows checking whether a
word
position is congruent to r modulo q, for some divisor q and remainder
r. If we allow the more powerful modulo counting quantifiers of
Straubing, Therien and Thomas (we call this two-variable fragment
FOMOD^{2}[<,successor]), satisfiability becomes
Expspace-complete. A
more general counting quantifier, FOCOUNT^{2}[<,successor],
makes the logic undecidable.

7.

Two-variable logic over
countable linear orderings, with

Amaldev
Manuel,

MFCS 2016.
(

pdf,

full,

abstract)

** Abstract **:
We
study the class of
languages of finitely-labelled countable linear orderings definable in
two-variable first-order logic. We give a number of characterisations,
in particular an algebraic one in terms of circle monoids, using
equations. This generalises the corresponding characterisation, namely
variety DA, over finite words to the countable case. A corollary is
that the membership in this class is decidable: for instance given an
MSO formula it is possible to check if there is an equivalent
two-variable logic formula
over countable linear orderings. In addition, we prove that the
satisfiability problems for two-variable logic over arbitrary,
countable, and scattered linear orderings are
Nexptime-complete.

6.

Limited Set quantifiers
over
Countable Linear Orderings, with

Thomas
Colcombet,

ICALP 2015.
(

pdf,

full,

abstract)

** Abstract **:
In
this paper, we study several sublogics of monadic second-order logic
over countable linear orderings, such as first-order logic, first-order
logic on cuts, weak monadic second-order logic, weak monadic
second-order logic with cuts, as well as fragments of monadic
second-order logic in which sets have to be well ordered or scattered.
We give decidable algebraic characterizations of all these logics and
compare their respective expressive power.

5.

Counting quantifiers
and
linear arithmetic on word models, with

Kamal Lodaya,

Asian
Logic Conference (ALC), 2014.
(

pdf)

4.

On lower bounds for
multiplicative circuits and linear circuits in
noncommutative domains, with

V.
Arvind and

S. Raja,
CSR, 2014.
(

pdf,

abstract)

** Abstract **:
In
this paper we show some lower bounds for the size of multiplicative
circuits computing multi-output functions in some noncommutative
domains like monoids and finite groups. We also introduce and study a
generalization of linear circuits in which the goal is to compute **MY**
where **Y** is a vector of indeterminates and **M** is a matrix
whose
entries come from noncommutative rings. We show some lower
bounds in this setting as well.

3.

Non-definability of
Languages by Generalized First-order Formulas
over (N, +), with

Andreas
Krebs,

LICS 2012.
(

pdf,

full,

abstract)

** Abstract **:
We
consider first-order logic with monoidal quantifiers. We show that all
languages with a neutral letter, definable using the addition numerical
predicate are also definable with the order predicate as the only
numerical predicate. Since we prove this result for arbitrary subsets
of the monoidal quantifiers, the following holds in the presence of a
neutral letter: For aperiodic monoids, we get the result of Libkin that
FO[<, +] collapses to FO[<]; For solvable monoids, we get the
result of Roy and Straubing that FO+MOD[<, +] collapses to
FO+MOD[<]; For cyclic groups, we answer an open question of Roy and
Straubing, proving that MOD[<, +] collapses to MOD[<].
All these results can be viewed as collapse results for the uniformity
of constant depth circuits, and in this sense as a separation result
for very uniform circuit classes. For example we separate
FO[<,+]-uniform CC^{0} from FO[<,+]-uniform ACC^{0}.

2.

Expressive Completeness
for
LTL With Modulo Counting and Group
Quantifiers,

Electronic
Notes in Theoretical Computer Science
(ENTCS),
2011.
(

pdf,

abstract)

** Abstract **:
Kamp
showed that linear temporal logic is expressively complete for
first order logic over words. We give a Gabbay style proof to show
that linear temporal logic extended with modulo counting and group
quantifiers (introduced by Baziramwabo,McKenzie,Th\'erien) is
expressively complete for first order logic with modulo counting
(introduced by Straubing, Th\'erien, Thomas) and
group quantifiers (introduced by Barrington, Immerman,
Straubing).

1.

LTL can be more succinct,
with

Kamal Lodaya,

ATVA 2010.
(

pdf,

abstract)

** Abstract **:
It
is well known that modelchecking and satisfiability of Linear Temporal
Logic (LTL) are Pspace-complete. Wolper showed that
with grammar operators, this result can be extended to increase the
expressiveness of the logic to all regular languages. Other ways of
extending the expressiveness of LTL using modular and group modalities
have been explored by Baziramwabo, McKenzie and Th\'erien, which are
expressively complete for regular languages recognized by solvable
monoids and for all regular languages, respectively. In all the papers
mentioned, the numeric constants used in the modalities are in unary
notation. We show that in some cases (such as the modular and symmetric
group modalities) we can use numeric constants in binary notation,
and still maintain the Pspace upper bound. Adding modulo counting to
LTL[F] (with just the unary future modality) already makes the logic
Pspace-hard. We also consider a restricted logic which allows only
the modulo counting of length from the beginning of the word. Its
satisfiability is Sigmathree-complete.

** Teaching**

One week course on

Descriptive Complexity Theory: An
Introduction, Indian School on Logic and its
Applications
(ISLA), PSG college of Technology, Coimbatore in March 2016

An one semester course titled

Verification
for graduate and undergraduate students in Wilhelm-Schickard-Institute
for Informatik, University of Tubingen, Germany, from March 2012 to
August 2012

** Abstract **:
We
study the class of
languages of finitely-labelled countable linear orderings definable in
two-variable first-order logic. We give a number of characterisations,
in particular an algebraic one in terms of circle monoids, using
equations. This generalises the corresponding characterisation, namely
variety DA, over finite words to the countable case. A corollary is
that the membership in this class is decidable: for instance given an
MSO formula it is possible to check if there is an equivalent
two-variable logic formula
over countable linear orderings. In addition, we prove that the
satisfiability problems for two-variable logic over arbitrary,
countable, and scattered linear orderings are
Nexptime-complete.

** Interesting Links **

Computer science
bibliography collections
Computer science conference timelines
Erdos
Number