/usr/include/geos/index/ is in libgeos++-dev 3.5.0-1ubuntu2.
This file is owned by root:root, with mode 0o755.
..
/usr/include/geos/index/ItemVisitor.h
/usr/include/geos/index/SpatialIndex.h
/usr/include/geos/index/bintree/
/usr/include/geos/index/bintree/Bintree.h
/usr/include/geos/index/bintree/Interval.h
/usr/include/geos/index/bintree/Key.h
/usr/include/geos/index/bintree/Node.h
/usr/include/geos/index/bintree/NodeBase.h
/usr/include/geos/index/bintree/Root.h
/usr/include/geos/index/chain/
/usr/include/geos/index/chain/MonotoneChain.h
/usr/include/geos/index/chain/MonotoneChainBuilder.h
/usr/include/geos/index/chain/MonotoneChainOverlapAction.h
/usr/include/geos/index/chain/MonotoneChainSelectAction.h
/usr/include/geos/index/intervalrtree/
/usr/include/geos/index/intervalrtree/IntervalRTreeBranchNode.h
/usr/include/geos/index/intervalrtree/IntervalRTreeLeafNode.h
/usr/include/geos/index/intervalrtree/IntervalRTreeNode.h
/usr/include/geos/index/intervalrtree/SortedPackedIntervalRTree.h
/usr/include/geos/index/quadtree/
/usr/include/geos/index/quadtree/DoubleBits.h
/usr/include/geos/index/quadtree/IntervalSize.h
/usr/include/geos/index/quadtree/Key.h
/usr/include/geos/index/quadtree/Node.h
/usr/include/geos/index/quadtree/NodeBase.h
/usr/include/geos/index/quadtree/Quadtree.h
/usr/include/geos/index/quadtree/Root.h
/usr/include/geos/index/strtree/
/usr/include/geos/index/strtree/AbstractNode.h
/usr/include/geos/index/strtree/AbstractSTRtree.h
/usr/include/geos/index/strtree/Boundable.h
/usr/include/geos/index/strtree/Interval.h
/usr/include/geos/index/strtree/ItemBoundable.h
/usr/include/geos/index/strtree/SIRtree.h
/usr/include/geos/index/strtree/STRtree.h
/usr/include/geos/index/sweepline/
/usr/include/geos/index/sweepline/SweepLineEvent.h
/usr/include/geos/index/sweepline/SweepLineIndex.h
/usr/include/geos/index/sweepline/SweepLineInterval.h
/usr/include/geos/index/sweepline/SweepLineOverlapAction.h