index = faiss.index_factory(d, "OPQ16_512,IVF1024,PQ16")
co = faiss.GpuClonerOptions()
co.useFloat16 = False
co.usePrecomputed = False
co.indicesOptions = faiss.INDICES_CPU
index = faiss.index_cpu_to_gpu(res, 0, index, co)

index.train(xt)
del xt

index.add(xb)

faiss.write_index(index, 'faiss.index')  # error happens here

 If I want to reuse the trained model, could it be trained again? I found nothing like saveModel or loadModel method in the codes.

ncentroids = 1024
niter = 20
verbose = true
d = x.shape[1]
kmeans = faiss.Kmeans(d, ncentroids, niter, verbose)
kmeans.train (x)

D, I = kmeans.index.search(x, 1)

#!/bin/python3

import faiss
import numpy as np

def samples(d, count):
    samples = np.random.random((count, d)).astype('float32')
    samples[:, 0] += np.arange(count) / 1000.
    return samples

def make_index_1(d):
    nlist = 100
    quantizer = faiss.IndexFlatL2(d)
    index = faiss.IndexIVFFlat(quantizer, d, nlist, faiss.METRIC_L2)

    return index, quantizer

def make_index_2(d):
    nlist = 100
    quantizer = faiss.IndexFlatL2(d)
    index = faiss.IndexIVFFlat(quantizer, d, nlist, faiss.METRIC_L2)

    return index

d = 32
examples = samples(d, 10000)
index1, q = make_index_1(d)
index1.train(examples)
print('OK')

index2 = make_index_2(d)
index2.train(examples) # segfaults here
print('OK')

res = faiss.StandardGpuResources()

index = faiss.index_factory(d, "OPQ8,IVF4096,PQ8")

co = faiss.GpuClonerOptions()
co.useFloat16 = False
co.usePrecomputed = True
co.indicesOptions = faiss.INDICES_CPU

index = faiss.index_cpu_to_gpu(res, 0, index, co)  # error happends here

Faiss assertion !"don't know how to serialize this type of index"

++ -fPIC -m64 -Wall -g -O3  -msse4 -mpopcnt -fopenmp -Wno-sign-compare -Dnullptr=NULL -Doverride= -fopenmp -c AutoTune.cpp -o AutoTune.o  
g++ -fPIC -m64 -Wall -g -O3  -msse4 -mpopcnt -fopenmp -Wno-sign-compare -Dnullptr=NULL -Doverride= -fopenmp -c AuxIndexStructures.cpp -o AuxIndexStructures.o  
ar r libfaiss.a hamming.o utils.o IndexFlat.o IndexIVF.o IndexLSH.o IndexPQ.o IndexIVFPQ.o Clustering.o Heap.o VectorTransform.o index_io.o PolysemousTraining.o MetaIndexes.o Index.o ProductQuantizer.o AutoTune.o AuxIndexStructures.o
ar: creating libfaiss.a
g++ -o tests/demo_ivfpq_indexing -fPIC -m64 -Wall -g -O3  -msse4 -mpopcnt -fopenmp -Wno-sign-compare -Dnullptr=NULL -Doverride= -fopenmp tests/demo_ivfpq_indexing.cpp libfaiss.a -g -fPIC  -fopenmp /usr/lib/libopenblas.so.0
libfaiss.a(utils.o): In function `faiss::matrix_qr(int, int, float*)':
/root/DVA/faiss/utils.cpp:1215: undefined reference to `sgeqrf_'
/root/DVA/faiss/utils.cpp:1220: undefined reference to `sgeqrf_'
/root/DVA/faiss/utils.cpp:1223: undefined reference to `sorgqr_'
libfaiss.a(VectorTransform.o): In function `faiss::OPQMatrix::train(long, float const*)':
/root/DVA/faiss/VectorTransform.cpp:603: undefined reference to `sgesvd_'
/root/DVA/faiss/VectorTransform.cpp:594: undefined reference to `sgesvd_'
libfaiss.a(VectorTransform.o): In function `faiss::PCAMatrix::train(long, float const*)':
/root/DVA/faiss/VectorTransform.cpp:284: undefined reference to `ssyev_'
/root/DVA/faiss/VectorTransform.cpp:289: undefined reference to `ssyev_'
collect2: error: ld returned 1 exit status
make: *** [tests/demo_ivfpq_indexing] Error 1

cno=12 key=nprobe=4096 perf=0.9870 t=11.214 
Traceback (most recent call last):
  File "python/demo_auto_tune.py", line 168, in <module>
    fig.savefig('tmp/demo_auto_tune.png')
  File "/usr/lib/pymodules/python2.7/matplotlib/figure.py", line 1421, in savefig
    self.canvas.print_figure(*args, **kwargs)
  File "/usr/lib/pymodules/python2.7/matplotlib/backend_bases.py", line 2220, in print_figure
    **kwargs)
  File "/usr/lib/pymodules/python2.7/matplotlib/backends/backend_agg.py", line 510, in print_png
    filename_or_obj = open(filename_or_obj, 'wb')
IOError: [Errno 2] No such file or directory: 'tmp/demo_auto_tune.png'

index = faiss.IndexFlatL2(d)
index.add(xb)
faiss.write_index(index, PATH)

index = faiss.read_index(PATH)
xb[0] = ?

Real-life test
--------------

The following script extends the demo_sift1M test to several types of
indexes:

  python python/demo_auto_tune.py

It will cycle through a few types of indexes and find optimal
operating points. You can play around with the types of indexes.

python python/demo_auto_tune.py
Traceback (most recent call last):
  File "python/demo_auto_tune.py", line 19, in <module>
    import faiss
ImportError: No module named faiss

$ git clone https://github.com/facebookresearch/faiss.git
$ cd faiss
$ cp example_makefiles/makefile.inc.Mac.brew makefile.inc
$ brew install llvm
$ brew install swig
$ make
/usr/local/opt/llvm/bin/clang++ -o tests/demo_ivfpq_indexing -fPIC -m64 -Wall -g -O3 -msse4 -mpopcnt -fopenmp -Wno-sign-compare -Dnullptr=NULL -I/usr/local/opt/llvm/include -Doverride= tests/demo_ivfpq_indexing.cpp libfaiss.a -g -fPIC -fopenmp -L/usr/local/opt/llvm/lib -framework Accelerate
ld: warning: could not create compact unwind for __ZN5faiss9matrix_qrEiiPf: dwarf uses DW_CFA_GNU_args_size
ld: warning: could not create compact unwind for __ZN5faiss9PCAMatrix10prepare_AbEv: dwarf uses DW_CFA_GNU_args_size
ld: warning: could not create compact unwind for __ZNK5faiss11IndexShards6searchElPKflPfPl: dwarf uses DW_CFA_GNU_args_size
Undefined symbols for architecture x86_64:
  "std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >::size() const", referenced from:
      std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > std::operator+<char, std::char_traits<char>, std::allocator<char> >(char const*, std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > const&) in libfaiss.a(VectorTransform.o)
      faiss::PolysemousTraining::optimize_reproduce_distances(faiss::ProductQuantizer&) const in libfaiss.a(PolysemousTraining.o)
      faiss::PolysemousTraining::optimize_ranking(faiss::ProductQuantizer&, unsigned long, float const*) const in libfaiss.a(PolysemousTraining.o)
      std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > std::operator+<char, std::char_traits<char>, std::allocator<char> >(char const*, std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > const&) in libfaiss.a(MetaIndexes.o)
  "std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >::c_str() const", referenced from:
      faiss::PolysemousTraining::optimize_reproduce_distances(faiss::ProductQuantizer&) const in libfaiss.a(PolysemousTraining.o)
      faiss::PolysemousTraining::optimize_ranking(faiss::ProductQuantizer&, unsigned long, float const*) const in libfaiss.a(PolysemousTraining.o)
  "std::__cxx11::basic_stringstream<char, std::char_traits<char>, std::allocator<char> >::str() const", referenced from:
      faiss::IndexIVFPQ::set_typename() in libfaiss.a(IndexIVFPQ.o)
      faiss::IndexIVFPQR::set_typename() in libfaiss.a(IndexIVFPQ.o)
      faiss::IndexIVFFlat::set_typename() in libfaiss.a(IndexIVF.o)
      faiss::IndexFlat::set_typename() in libfaiss.a(IndexFlat.o)
      faiss::IndexRefineFlat::set_typename() in libfaiss.a(IndexFlat.o)
      faiss::IndexPQ::set_typename() in libfaiss.a(IndexPQ.o)
      faiss::MultiIndexQuantizer::set_typename() in libfaiss.a(IndexPQ.o)
      ...
  "std::allocator<char>::allocator()", referenced from:
      faiss::Index::Index(long, faiss::MetricType) in libfaiss.a(index_io.o)
      faiss::Index::Index(long, faiss::MetricType) in libfaiss.a(IndexIVF.o)
      faiss::Index::Index(long, faiss::MetricType) in libfaiss.a(IndexFlat.o)
      faiss::Index::Index(long, faiss::MetricType) in libfaiss.a(VectorTransform.o)
      faiss::Index::Index(long, faiss::MetricType) in libfaiss.a(IndexPQ.o)
      faiss::Index::Index(long, faiss::MetricType) in libfaiss.a(IndexLSH.o)
      faiss::Index::Index(long, faiss::MetricType) in libfaiss.a(MetaIndexes.o)
      ...
  "std::allocator<char>::~allocator()", referenced from:
      faiss::Index::Index(long, faiss::MetricType) in libfaiss.a(index_io.o)
      faiss::Index::Index(long, faiss::MetricType) in libfaiss.a(IndexIVF.o)
      faiss::Index::Index(long, faiss::MetricType) in libfaiss.a(IndexFlat.o)
      faiss::Index::Index(long, faiss::MetricType) in libfaiss.a(VectorTransform.o)
      faiss::Index::Index(long, faiss::MetricType) in libfaiss.a(IndexPQ.o)
      faiss::Index::Index(long, faiss::MetricType) in libfaiss.a(IndexLSH.o)
      faiss::Index::Index(long, faiss::MetricType) in libfaiss.a(MetaIndexes.o)
      ...
  "std::ostream::operator<<(int)", referenced from:
      faiss::IndexLSH::set_typename() in libfaiss.a(IndexLSH.o)
  "std::ostream::operator<<(unsigned long)", referenced from:
      faiss::IndexIVFPQ::set_typename() in libfaiss.a(IndexIVFPQ.o)
      faiss::IndexIVFPQR::set_typename() in libfaiss.a(IndexIVFPQ.o)
      faiss::IndexIVFFlat::set_typename() in libfaiss.a(IndexIVF.o)
      faiss::IndexPQ::set_typename() in libfaiss.a(IndexPQ.o)
      faiss::MultiIndexQuantizer::set_typename() in libfaiss.a(IndexPQ.o)
  "std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >::append(char const*)", referenced from:
      std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > std::operator+<char, std::char_traits<char>, std::allocator<char> >(std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >&&, char const*) in libfaiss.a(VectorTransform.o)
      std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > std::operator+<char, std::char_traits<char>, std::allocator<char> >(std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >&&, char const*) in libfaiss.a(MetaIndexes.o)
  "std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >::append(char const*, unsigned long)", referenced from:
      std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > std::operator+<char, std::char_traits<char>, std::allocator<char> >(char const*, std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > const&) in libfaiss.a(VectorTransform.o)
      std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > std::operator+<char, std::char_traits<char>, std::allocator<char> >(char const*, std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > const&) in libfaiss.a(MetaIndexes.o)
  "std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >::append(std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > const&)", referenced from:
      std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > std::operator+<char, std::char_traits<char>, std::allocator<char> >(char const*, std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > const&) in libfaiss.a(VectorTransform.o)
      std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > std::operator+<char, std::char_traits<char>, std::allocator<char> >(char const*, std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > const&) in libfaiss.a(MetaIndexes.o)
  "std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >::reserve(unsigned long)", referenced from:
      std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > std::operator+<char, std::char_traits<char>, std::allocator<char> >(char const*, std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > const&) in libfaiss.a(VectorTransform.o)
      std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > std::operator+<char, std::char_traits<char>, std::allocator<char> >(char const*, std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > const&) in libfaiss.a(MetaIndexes.o)
  "std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >::basic_string(std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >&&)", referenced from:
      std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > std::operator+<char, std::char_traits<char>, std::allocator<char> >(std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >&&, char const*) in libfaiss.a(VectorTransform.o)
      std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > std::operator+<char, std::char_traits<char>, std::allocator<char> >(std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >&&, char const*) in libfaiss.a(MetaIndexes.o)
  "std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >::basic_string(char const*, std::allocator<char> const&)", referenced from:
      faiss::Index::Index(long, faiss::MetricType) in libfaiss.a(index_io.o)
      faiss::Index::Index(long, faiss::MetricType) in libfaiss.a(IndexIVF.o)
      faiss::Index::Index(long, faiss::MetricType) in libfaiss.a(IndexFlat.o)
      faiss::Index::Index(long, faiss::MetricType) in libfaiss.a(VectorTransform.o)
      faiss::Index::Index(long, faiss::MetricType) in libfaiss.a(IndexPQ.o)
      faiss::Index::Index(long, faiss::MetricType) in libfaiss.a(IndexLSH.o)
      faiss::Index::Index(long, faiss::MetricType) in libfaiss.a(MetaIndexes.o)
      ...
  "std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >::basic_string(std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > const&)", referenced from:
      __ZNK5faiss5Index12get_typenameB5cxx11Ev in libfaiss.a(IndexIVFPQ.o)
      __ZNK5faiss5Index12get_typenameB5cxx11Ev in libfaiss.a(index_io.o)
      faiss::Index::Index(faiss::Index const&) in libfaiss.a(index_io.o)
      faiss::PolysemousTraining::PolysemousTraining(faiss::PolysemousTraining const&) in libfaiss.a(index_io.o)
      __ZNK5faiss5Index12get_typenameB5cxx11Ev in libfaiss.a(Index.o)
      __ZNK5faiss5Index12get_typenameB5cxx11Ev in libfaiss.a(IndexIVF.o)
      __ZNK5faiss5Index12get_typenameB5cxx11Ev in libfaiss.a(IndexFlat.o)
      ...
  "std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >::basic_string()", referenced from:
      std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > std::operator+<char, std::char_traits<char>, std::allocator<char> >(char const*, std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > const&) in libfaiss.a(VectorTransform.o)
      faiss::PolysemousTraining::PolysemousTraining() in libfaiss.a(PolysemousTraining.o)
      faiss::PolysemousTraining::PolysemousTraining() in libfaiss.a(PolysemousTraining.o)
      std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > std::operator+<char, std::char_traits<char>, std::allocator<char> >(char const*, std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > const&) in libfaiss.a(MetaIndexes.o)
  "std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >::~basic_string()", referenced from:
      faiss::IndexIVFPQ::set_typename() in libfaiss.a(IndexIVFPQ.o)
      faiss::IndexIVFPQR::set_typename() in libfaiss.a(IndexIVFPQ.o)
      faiss::PolysemousTraining::~PolysemousTraining() in libfaiss.a(IndexIVFPQ.o)
      faiss::Index::~Index() in libfaiss.a(index_io.o)
      faiss::IndexIVFFlat::set_typename() in libfaiss.a(IndexIVF.o)
      faiss::Index::~Index() in libfaiss.a(IndexIVF.o)
      faiss::IndexFlat::set_typename() in libfaiss.a(IndexFlat.o)
      ...
  "std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >::operator=(std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >&&)", referenced from:
      faiss::IndexIVFPQ::set_typename() in libfaiss.a(IndexIVFPQ.o)
      faiss::IndexIVFPQR::set_typename() in libfaiss.a(IndexIVFPQ.o)
      faiss::IndexIVFFlat::set_typename() in libfaiss.a(IndexIVF.o)
      faiss::IndexFlat::set_typename() in libfaiss.a(IndexFlat.o)
      faiss::IndexRefineFlat::set_typename() in libfaiss.a(IndexFlat.o)
      faiss::IndexPreTransform::set_typename() in libfaiss.a(VectorTransform.o)
      faiss::IndexPQ::set_typename() in libfaiss.a(IndexPQ.o)
      ...
  "std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >::operator=(std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > const&)", referenced from:
      faiss::Index::operator=(faiss::Index const&) in libfaiss.a(index_io.o)
  "std::__cxx11::basic_stringstream<char, std::char_traits<char>, std::allocator<char> >::basic_stringstream(std::_Ios_Openmode)", referenced from:
      faiss::IndexIVFPQ::set_typename() in libfaiss.a(IndexIVFPQ.o)
      faiss::IndexIVFPQR::set_typename() in libfaiss.a(IndexIVFPQ.o)
      faiss::IndexIVFFlat::set_typename() in libfaiss.a(IndexIVF.o)
      faiss::IndexFlat::set_typename() in libfaiss.a(IndexFlat.o)
      faiss::IndexRefineFlat::set_typename() in libfaiss.a(IndexFlat.o)
      faiss::IndexPQ::set_typename() in libfaiss.a(IndexPQ.o)
      faiss::MultiIndexQuantizer::set_typename() in libfaiss.a(IndexPQ.o)
      ...
  "std::__cxx11::basic_stringstream<char, std::char_traits<char>, std::allocator<char> >::~basic_stringstream()", referenced from:
      faiss::IndexIVFPQ::set_typename() in libfaiss.a(IndexIVFPQ.o)
      faiss::IndexIVFPQR::set_typename() in libfaiss.a(IndexIVFPQ.o)
      faiss::IndexIVFFlat::set_typename() in libfaiss.a(IndexIVF.o)
      faiss::IndexFlat::set_typename() in libfaiss.a(IndexFlat.o)
      faiss::IndexRefineFlat::set_typename() in libfaiss.a(IndexFlat.o)
      faiss::IndexPQ::set_typename() in libfaiss.a(IndexPQ.o)
      faiss::MultiIndexQuantizer::set_typename() in libfaiss.a(IndexPQ.o)
      ...
  "std::__throw_length_error(char const*)", referenced from:
      std::vector<std::vector<unsigned char, std::allocator<unsigned char> >, std::allocator<std::vector<unsigned char, std::allocator<unsigned char> > > >::_M_check_len(unsigned long, char const*) const in libfaiss.a(IndexIVFPQ.o)
      std::vector<long, std::allocator<long> >::_M_check_len(unsigned long, char const*) const in libfaiss.a(IndexIVFPQ.o)
      std::vector<unsigned char, std::allocator<unsigned char> >::_M_check_len(unsigned long, char const*) const in libfaiss.a(IndexIVFPQ.o)
      std::vector<float, std::allocator<float> >::_M_check_len(unsigned long, char const*) const in libfaiss.a(IndexIVFPQ.o)
      std::vector<float const*, std::allocator<float const*> >::_M_check_len(unsigned long, char const*) const in libfaiss.a(IndexIVFPQ.o)
      std::vector<float, std::allocator<float> >::_M_check_len(unsigned long, char const*) const in libfaiss.a(index_io.o)
      std::vector<int, std::allocator<int> >::_M_check_len(unsigned long, char const*) const in libfaiss.a(index_io.o)
      ...
  "std::basic_ostream<char, std::char_traits<char> >& std::operator<<<std::char_traits<char> >(std::basic_ostream<char, std::char_traits<char> >&, char const*)", referenced from:
      faiss::IndexIVFPQ::set_typename() in libfaiss.a(IndexIVFPQ.o)
      faiss::IndexIVFPQR::set_typename() in libfaiss.a(IndexIVFPQ.o)
      faiss::IndexIVFFlat::set_typename() in libfaiss.a(IndexIVF.o)
      faiss::IndexFlat::set_typename() in libfaiss.a(IndexFlat.o)
      faiss::IndexRefineFlat::set_typename() in libfaiss.a(IndexFlat.o)
      faiss::IndexPQ::set_typename() in libfaiss.a(IndexPQ.o)
      faiss::MultiIndexQuantizer::set_typename() in libfaiss.a(IndexPQ.o)
      ...
  "std::basic_ostream<char, std::char_traits<char> >& std::operator<<<std::char_traits<char> >(std::basic_ostream<char, std::char_traits<char> >&, char)", referenced from:
      faiss::IndexRefineFlat::set_typename() in libfaiss.a(IndexFlat.o)
  "std::basic_ostream<char, std::char_traits<char> >& std::operator<<<char, std::char_traits<char>, std::allocator<char> >(std::basic_ostream<char, std::char_traits<char> >&, std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > const&)", referenced from:
      faiss::IndexIVFPQ::set_typename() in libfaiss.a(IndexIVFPQ.o)
      faiss::IndexIVFPQR::set_typename() in libfaiss.a(IndexIVFPQ.o)
      faiss::IndexIVFFlat::set_typename() in libfaiss.a(IndexIVF.o)
      faiss::IndexRefineFlat::set_typename() in libfaiss.a(IndexFlat.o)
ld: symbol(s) not found for architecture x86_64
clang-4.0: error: linker command failed with exit code 1 (use -v to see invocation)
make: *** [tests/demo_ivfpq_indexing] Error 1

root@428311598e54:/tmp/faiss-master# ls /opt/intel/compilers_and_libraries_2016.3.210/linux/mkl/lib/intel64/{libmkl_core.so,libmkl_avx2.so}
/opt/intel/compilers_and_libraries_2016.3.210/linux/mkl/lib/intel64/libmkl_avx2.so
/opt/intel/compilers_and_libraries_2016.3.210/linux/mkl/lib/intel64/libmkl_core.so