Table 13 Most frequent category including PR (Category labelled by 2020)

(COVID-19, SARS-CoV-2, Coronavirus)

(Aristotle, Plato, Rome)

(Aeroacoustics, Numerical simulation, Noise)

(MoS, 2D materials, transition metal dichalcogenides)

(Hippocampus, Memory, Episodic memory)

(Slavery, Spain, History)

(Liquefaction, Sand, Anisotropy)

(Enhanced oil recovery, Interfacial tension, Surfactant)

(AHP, TOPSIS, Decision making)

(Electronic structure, Density functional theory, High pressure)

(MHD, Heat transfer, Nanofluid)

(Truth, Explanation, Metaphysics)

(Item response theory, item response theory, Reliability)

(Microstructure, Retained austenite, Mechanical properties)

(Distributed video coding, Distributed source coding, Multiple description coding)

(Rock mechanics, Acoustic emission, Numerical simulation)

(Internal waves, Climate change, Sea level)

(Medicinal plants, Ethnobotany, Antioxidant)

(Kalman filter, Target tracking, Particle filter)

(Psychoanalysis, Trauma, Countertransference)

(Metacognition, Learning, Problem solving)

(Demand response, Smart grid, Optimization)

(Rats, pigeons, Rat)

(Precipitation, Rainfall, Remote sensing)

(Shale gas, Shale, Ordos Basin)

(Concrete, Compressive strength, Size effect)

(Motor control, Motor learning, Human)

(Heavy oil, Enhanced oil recovery, SAGD)

(Power system, Voltage stability, FACTS)

(Sequence stratigraphy, Holocene, Sediment transport)

(Motion, Visual cortex, Vision)

(Hydraulic fracturing, Hydraulic fracture, Numerical simulation)

(Shakespeare, Gender, Reformation)

(Heat transfer, Flow boiling, Boiling)

(Silicon, Solar cell, Nanowires)

(Density functional theory, DFT, density functional theory)

(Polyphasic taxonomy, Taxonomy, 16S rRNA gene)

(Black holes, Black Holes, Classical Theories of Gravity)

(Influenza, Influenza virus, Avian influenza)

(Elections, Political parties, European Union)

(Prejudice, Attitudes, Social identity)

(Dielectric properties, Ceramics, Ferroelectrics)

(Thermoelasticity, Generalized thermoelasticity, Laplace transform)

(Microstructure, Mechanical properties, Martensitic transformation)

(Spintronics, Magnetic tunnel junction, MRAM)

(Social media, Advertising, Consumer behaviour)

(Finite element method, Finite elements, Fluid-structure interaction)

(Complex networks, Complex network, Social networks)

(Pile, Piles, Pile foundation)

(Water splitting, Photocatalysis, water splitting)