Negative exponent: PV panel cost in dollars per peak watt; lithium battery cost is $ per kWh. Possibly: reverse osmosis water purification, proton exchange mem

Negative exponent: PV panel cost in dollars per peak watt; lithium battery cost is $ per kWh. Possibly: reverse osmosis water purification, proton exchange membrane electrolysis, smelting of iron using hydrogen or direct electric arc reduction (early days yet). Total fertility rate on a per-country basis. Positive exponent: number of staff per student in tertiary education institutions. OK, that's a bit tongue in cheek. A bit.