Read this for an excellent SG on trading for more tips:
https://forums.civfanatics.com/threads/training-game-emphasizing-trading-emperor.106205/ That thread was one of the most valuable teachers I had.
Thanks for pointing that out Blackhat! I kind of wished they had gone to the modern age with it and traded around for modern techs also. It's given me some ideas.
1. I've lately tried to use gpt to purchase techs for 2-fers early on in some Sid starts. The reason for this lies in that you can use the tax collector trick to increase your gpt (you change all of your citizens to tax collectors, make trades and then put them back onto tiles... Moonsinger's 88k histographic game shows clear evidence of using the tax collector trick in her 1000 BC save... so does my lesser scoring game which should come in the update). But, I've had a problem in that even when I can manage to make a trade I can't always seem to get my gpt back on the second deal. I've finally realized that since you can always get a lump sum back, when doing an n-fer use as much of your lump sum as you can on the first deal and then get it back!
2. The best gold to gpt ratio is 18 gold to 1 gpt for a 20 turn deal. So, I think you end spending more money over the long turn if you use gpt. But, you never know what the AI will do with the gpt in a turn. They might have a negative cash flow and thus giving them more gpt will go nowhere. But, if they have a lump sum, they can use that lump sum for techs. Thus, draining the AI when doing trades early can help. The gold in the bank might also help towards a steal.
3. If you end up doing/having multiple trades available in a turn... say at the age change with scientific opponents, you might use MapStat in the following way.
A. You do the first set of deals.
B. Save the game.
C. Load the save using MapStat to see what all the AIs have available at the same time.
4. Keeping track of who knows whom and who doesn't might help things out.
5. It's not mentioned in that thread, but you might use wonder initiation to clue you into when you might have the ability to pull off an n-fer (in addition to change of age). Here's an example that I add in my recent game, which I wish I had made screenshots of, but it kind of happened as a surprise to me that I pulled it off at the time.
I had Alphabet and Ceremonial Burial. So, I couldn't see past Writing and Mysticism. One tribe had started building the Temple of Artemis. Another had started on the Great Lighthouse. Another tribe had The Wheel and no one had it. So, I managed to purchase The Wheel, and Writing and pulled off basically a 3-fer.
6. So, let's say you end up a bit behind and can't tell if the AIs have a particular tech yet or not. Let's say you don't have Masonry, but you do have Writing (doesn't sound like all that great of a game to me, but I digress). Do they have Mathematics? Well, if you investigate their cities and they have a catapult or a marketplace in that city, then they have mathematics.
You can also tell that they have a resource without you having access to that resource itself.
So, let's see if I can make a list of all of the techs that you might have the ability to tell without any wonders corresponding to those techs.
Ancient Age:
1. There's no need for anything after Writing, since you can see Literature, Code of Laws, Map Making, and Philosophy directly, except The Republic. And you might check the Military Advisor for that.
2. You might check the Military Advisor to see if someone has Monarchy also.
3. Sworsdmen might be in a city if you don't have Bronze Working and want to know if they have Iron Working.
4. They might have a catapult if you can't see Mathematics.
5. If you can't see Currency, they might have a market in a city.
6. If they have or are building horseman/their UU, then they have horseback riding.
So, in the Ancient Age there exists only one tech which you have no chance of determining if the AI have or not if you don't have the previous tech: Writing.
Middle Ages:
1. If the city has a cathedral they will have Monotheism.
2. If they have a trebuchet, or are building one, then they have Engineering.
3. If the city has a bank, they will have Banking.
4. If they have saltpeter in a city, then they have Gunpowder. Thus, you might manage to nail down when the AI gets gunpowder without having Invention.
5. If they have a cannon or coastal fortress in the city, then they have Metallurgy.
6. The military advisor might tell you when they have Democracy.
So, the only techs in the middle ages you have no chance of knowing when the AI learns are Chemistry, Physics, and the Printing Press. But, those are techs that the AI tends to wait a while to research.
Industrial Age:
1. If they have a rifleman or are training one, then they have Nationalism.
2. The military advisor might tell you when they have Fascism.
3. The military advisor might tell you when they have Communism.
4. If they have or are building and Ironclad, they have Ironclads.
5. If they have an Intelligence Agency or the SPHQ, then they have Espionage.
6. If they have coal, then they have Steam Power.
7. If they have a hospital or are building one, then they have Sanitation.
8. If they have a Stock Exchange or are building one, then they have The Corporation.
9. If they have or are building artillery proper, an infantry, or have rubber, then they have Replacable Parts.
10. If they have oil, then they have Refining.
11. If they have or are building a cruiser, destroyer, or transport, then they have Combustion.
12. If they have or are building an airport, fighter, bomber, flak, or an expired Colossus (what's their government... how much commerce should they have without it?), then they have Flight. Also, if they've have an airfield in their territory, then they have Flight.
13. If they have a marine, then they have amphibious war.
14. If they have or are building a battleship, carrier, sub, or commerical dock, then they have Mass Production.
15. If they have or are building a tank, or panzer, then they have Motorized Transportation.
16. If they have a radar tower in their territory, then they have Advanced Flight. Also, if they have or are building a helicopter paratrooper, then they have Advanced Flight.
17. You might also find some irrigation by a worker which makes no sense unless the Electricity.
So, I think, that leaves Medicine, Steel, and Atomic Theory as the only Industrial Age techs which you can't tell when the AI has if you're more than one tech behind.
Modern Age:
1. If they have or are building a mass transit system or solar plant, then they have Ecology.
2. If they have or are building a tow infrantry, a SAM missile battery, a cruise missile, a jet fighter, a mobile SAM, a TOW infantry, an F-15 or have aluminum, then they have Rocketry.
3. If they have or are building a Recylcing Center, then they have Recycling.
4. If they have or are building a nuclear plant, then they have Nuclear Power.
5. If they have or are building a modern armor, a modern paratrooper, the SS exterior casing, or the SS storage supply, then they have Robotics.
6. If they have or are building the SS planetary party lounge, then they have The Laser.
7. If they have or are building a Stealth Bomber or Stealth Fighter, then they have Stealth.
8. If they have or are building the SS fuel cells or the SS life support, then they have The Superconductor.
9. If they have or are building an ICBM or the SS thrusters, then they have Satellites.
10. If they have or are building a Manufacturing Plant, an SS stasis chamber, an AEGIS Cruiser, or radar artillery, then they have Robotics.
11. If they have or are building strategic missile defense, then they have integrated defense.
So, that only leaves smart weapons in the modern era as when you have no chance of determining when they have it.
The above kind of makes me think of a variant where you ONLY research those techs which you couldn't tell when the AI picks up.
Additionally, maybe it suggests one of the following two possibilities.
1. Forget stealing. The AI has a maximum research rate of 4 turns per tech. So, even if you can't tell when a tech appears using MapStat, maybe you can time out an n-fer possibility via some careful investigation of cities at the right time. Then you buy one tech to get to the n-fer techs, pull off the n-fer and get your cash/gpt back... or at least as much as you can. Looking at one of my modern age saves, it seems that stealing tech does go up quite a bit, but investigating cities doesn't. And you sometimes have cheaper cities which you can investigate. Checking two different saves of one of my old games... one modern and one medieval it seems that the cost of investigating cities depends on city size. That also perhaps leads to how the expansionist trait/building some explorers/signing ROPs/getting world maps can play to your advantage, since if you can find the smallest city of a tribe (which has connection to their capital if looking for a resource), then you might find the cheapest place to investigate. There's also the coastal aspect of the city.
2. If you're going to steal, other than during age-change, if you believe that an n-fer might be possible after doing some investigations, and you don't have enough to purchase two techs before selling one, but you do believe you have enough cash/gpt to pull it off if you steal a tech, then steal from the weakest tribe with one of the intermediate techs. Or one of the more distant tribes. That way, if you steal fails and they declare war, you'll have a greater probability of surviving.
Now if I can find out again how to calculate the cost of tech...