݄ ৽Ұ ʢژେֶཧղੳݚڀॴʣ
ཁ
༗ཧମQͷΑ͏ͳʮମʯͱɺෳͷυʔφπͷද໘Λ߹ମͤͨ͞Α͏ͳܗΛͨ͠ί ϯύΫτͳʮҐ૬ۂ໘ʯҰݟͯ͠શ͘ҟ࣭ͳֶతରͰ͋ΓɺॳతͳՄɺͭ
·ΓɺʮՃݮআʯ͕Մೳͳֶతରͱͯ͠ͷߏͷཧ͔Βݟͯతʹؔ࿈͚Δ
͜ͱ͍͠ɻ͔͠͠ମͷ֦େମͷରশੑΛهड़͢ΔʮઈରΨϩΞ܈ʯͱɺίϯύΫτ ͳҐ૬ۂ໘ͷ༗ݶ࣍ͷඃ෴ͷରশੑΛ౷੍͢Δʮ෭༗ݶجຊ܈ʯΛ௨ͯ྆͠ऀΛվΊͯோ
ΊͯΈΔͱɺʮೋ࣍ݩతͳ܈తབྷ·Γ߹͍ʯͱ͍͏ܗͰେมʹڵຯਂ͍ߏతͳྨࣅੑ
͕ු͔ͼ্͕ͬͯ͘ΔɻຊߘͰ༷ʑͳଆ໘ʹ͓͚Δ͜ͷछͷྨࣅੑʹযΛͯͳ͕Βɺ
ମͱҐ૬ۂ໘ͷجૅతͳཧʹ͍ͭͯղઆ͢Δɻ
࣍
§1. ମͷͱ֦େ
§1.1. ମͷఆٛ
§1.2. ૉʹਵ͢Δ༷ʑͳڑͷ֓೦
§1.3. ֦େମͱΨϩΞ܈
§2. Ґ૬ۂ໘্ͷྠମͱඃ෴
§2.1. ίϯύΫτͳҐ૬ۂ໘ͷఆٛͱछ
§2.2. Ґ૬ۂ໘ͷجຊ܈
§2.3. Ґ૬ۂ໘ͷඃ෴ͱඃ෴ม܈
§3. ίϗϞϩδʔʹΑΔʮ࣍ݩʯͷఆٛ
§3.1. Ґ૬زԿʹ͓͚ΔίϗϞϩδʔ࣍ݩ
§3.2. Ґ૬ۂ໘ͷίϗϞϩδʔ࣍ݩ
§3.3. ମͷίϗϞϩδʔ࣍ݩ
§4. ମͱҐ૬ۂ໘ͷʮབྷ·Γ߹͍ͷݱʯɿମ্ͷۂઢ
§4.1. ମ্ͷۂతۂઢ
§4.2. ෭༗ݶجຊ܈ͷઈରΨϩΞ܈ͷ࣮ͳ֎࡞༻
1
§1. ମͷͱ֦େ
§1.1. ମͷఆٛ
Ͱɺ
ࣗવ N = {0,1,2, . . .}
Z = {0,±1,±2, . . .}
༗ཧ Q = {a/b | a, b∈Z, b= 0}
⊆ R = {a | a ࣮} ͷΑ͏ͳʮී௨ͷʯͷଞʹɺత
Q = {x ∈ C | xn+cn−1xn−1+. . . c1x+c0 = 0; c0, c1, . . . , cn−1 ∈Q}
⊆ C = {a+bi | a, b ∈R}
͕ݚڀͷରʹͳΔɻ্هͷࣜͰɺ࣮ମRෳૉମC ͷΑ͏ͳʮೖΕʯຊ
ಋೖ͢Δඞཁͳ͘ɺநతͳܗͰQΛఆٛͨ͠Γɺߏͨ͠Γ͢Δ͜ͱՄೳͰ͋Δ
͕ɺ͜͜Ͱɺ؆୯ͷͨΊɺೖΕΛ༻͍Δ͜ͱʹ͢ΔɻࢹΛ༗ཧʹݶఆͤͣʹ
త·Ͱ͛Δ ʢʹ͛͟ΔΛಘͳ͍ͱͰ͍͏͖ͩΖ͏͔ʣ͜ͱʹ༷ʑͳཧ༝എ ܠ͕͋Δ͕ɺڪΒ͘Ұ൪جຊతͳཧ༝ɺྫ͑༗ཧͷੑ࣭ʹ͔͠ڵຯ͕ͳ͍ͱ͍͏ཱ
ʹཱͬͯɺͦͷੑ࣭ͷதʹɺ
తશମΛߟ͢Δ͜ͱʹΑͬͯॳΊͯ͡Δ͜ͱ͕ՄೳʹͳΔ
ͷ͕ࢁଘࡏ͢Δ͜ͱʹ͋Δɻ͜͏͍ͬͨݱɺຊߘͰऔΓ্͛Δ༧ఆͷʮ܈తͳʯ ཧͷதͰ͔ͳΓຊ࣭తͳܗͰݱΕΔͷͰ͋Δɻ
తશମʢʹQʣͩͱɺߏ͕ෳࡶա͗ͯखʹෛ͑ͳ͍ͱ͜Ζ͕͋ΔͨΊɺͦͷத ͷʮ༗ݶతͳ෦ʯΛऔΓग़ͯ͠͡Δ͜ͱ͕ଟ͍ɻ͜ͷΑ͏ͳʮ༗ݶతͳ෦ʯͷ͜ͱ ΛʮମʯͱݺͿ͕ɺͦͷఆٛΛड़Δલʹɺ·ͣʮ෦ମʯͷఆٛΛड़Δඞཁ͕͋Δɻ ମQʢ͋Δ͍ɺCRʣͷதͷʮ෦ମʯͱɺͦͷମͷதͷ෦ू߹K Ͱ͔͋ͬͯ
ͭՃݮআͰด͍ͯ͡Δͷͷ͜ͱΛ͍͏ɻʮମʯ
F ⊆ Q
ͱɺQͷ෦ମͰ͔͋ͬͯͭ͋Δ༗ݶू߹E ⊆ F ΛؚΉQͷ෦ମͷதͰ࠷খͷ
ͷͰ͋Δମͷ͜ͱΛ͍͏ɻ͜ͷΑ͏ͳঢ়گͰɺF ʢQͷ্ͰʣE ʹΑͬͯੜ͞ΕΔ ͱ͍͍ɺ·ͨEͷݩͷ͜ͱΛʮੜݩʯͱݺͿɻ
ମͷʮ۩ମྫʯແʹ͋Δ͕ɺྫ͑
Q; Q(√
−1); Q(√3
2); Q(√3 2,√
−3)
ʢͨͩ͠ɺׅހͰྻڍ͞Ε͍ͯΔੜݩͰ͋Δʣͯ͢ମͰ͋ΔɻҰํɺԁप
πΛੜݩͱ͢ΔΑ͏ͳRͷ෦ମ
Q(π) ⊆ R
ମʹͳΒͳ͍ɻͳͥͳΒɺΑ͘ΒΕ͍ͯΔΑ͏ʹπ ʮӽʯͰ͋ΔͨΊɺ
తͷఆٛʹ͋ΒΘΕΔΑ͏ͳ༗ཧͷଟ߲ࣜͷࠜʹͳΓ͑ͳ͍͔ΒͰ͋Δɻ
ମͷݚڀͰɺମͷ༷ʑͳੑ࣭Λʹ͢Δ͕ɺڪΒ͘࠷جຊతͳੑ࣭ͷҰͭɺ ʢྫ͑༗ཧମQ্ͰʣʮΨϩΞʯ (Galois) Ͱ͋Δ͔Ͳ͏͔ͱ͍͏ੑ࣭Ͱ͋ΔɻମF
͕ʢQ্ʣΨϩΞͰ͋ΔͱɺF ͷҙͷݩx∈F ʹରͯ͠ɺతͷఆٛʹ͋ΒΘΕ ΔΑ͏ͳ༗ཧͷଟ߲ࣜf(T) =Tn+cn−1Tn−1+. . . c1T +c0 = 0ͱͯ͠ɺx Λࠜʹ
࣋ͭͷͷதͰ࣍n͕࠷খʹͳΔΑ͏ͳͷʹʮxͷQ্ͷ࠷খଟ߲ࣜʯΛ࠾ͬͨͱ͖ɺ
ͦͷଟ߲ࣜͷࠜͷ͕ͯ͢F ʹೖΔ
ͱ͍͏݅Λຬͨ͢ͷͷ͜ͱΛ͍͏ɻ࣮ɺ͜ͷx∈F ʹର͢Δ݅ɺͯ͢ͷੜ
ݩʹରͯ֬͠ೝ͢ΕेͰ͋Δ͜ͱ༰қʹূ໌Ͱ͖Δɻ ઌఔͷʮ۩ମྫʯͷ͏ͪɺ
Q; Q(√
−1); Q(√3 2,√
−3)
ΨϩΞͰ͋Δ͕ɺQ(√3
2)ΨϩΞͰͳ͍ɻ͜ͷࣄ࣮ɺQͷ߹ʹ໌Β͔Ͱ͋Δ͕ɺ Q(√
−1)ͷ߹ɺੜݩi def= √
−1͕ຬͨ͢ଟ߲ࣜf(T) =T2+ 1ͷࠜͷू߹{±i} ͱ ͳΔͨΊɺཱ͕݅͢Δ͜ͱ͕ͪʹ͔Δɻ·ͨɺମQ(√3
2,√
−3)ͷ߹ɺ
ω def= −1 +√
−3
2 = e2πi/3 ∈ Q(√3 2,√
−3)
ͱஔ͘ͱɺ√
−3͕ຬͨ͢ଟ߲ࣜf(T) =T2+ 3ͷࠜͷू߹{±√
−3}Ͱ͋Γɺ√3
2͕ຬͨ
͢ଟ߲ࣜf(T) =T3−2ͷࠜͷू߹
{ √3
2, ω·√3
2, ω2·√3
2} ⊆ Q(√3 2,√
−3)
ͱͳΔͨΊɺ͜ͷମʹ͍ͭͯॴͷཱ͕݅͢ΔͷͰ͋ΔɻҰํɺQ(√3
2)࣮ମ Rͷ෦ମͰ͋ΔͨΊɺf(T) =T3−2ͷࠜͷҰͭͰ͋Δω·√3
2ʢʹڏ෦θϩͰͳ͍ͨ
Ίɺ࣮ʹͳΒͳ͍ʂʣ͕ମQ(√3
2)ʹؚ·Εͳ͍͜ͱͪʹ֬ೝͰ͖Δɻଈͪɺ
ମQ(√3
2)ΨϩΞʹͳΒͳ͍ͱ͍͏͜ͱͰ͋Δɻ
͜Ε·Ͱग़͖ͯͨମͷྫͰɺෳͷੜݩͰಛఆ͞Εͨͷ͕͋ͬͨɺ࣮ɺ
ҙͷମF ʹରͯ͠ɺͦͷதͷʮదʯͳݩα ∈F ΛબͿͱɺF α͚ͩͰੜ͞ΕΔɺ
ͭ·Γ
F = Q(α)
ͱͳΔ͜ͱ͕ΒΕ͍ͯΔɻ͜ͷͱ͖ɺαͷQ্ͷ࠷খଟ߲ࣜf(T) ͷ࣍Λ [F :Q]
ͱද͠ɺମF ͷ࣍ͱݺͿͷͰ͋Δɻ͜ͷΑ͏ʹఆٛ͞Εͨʮ[F :Q]ʯҰݟͯ͠α f(T)ͷબʹґଘ͢ΔΑ͏ʹݟ͑Δ͕ɺ࣮ґଘ͠ͳ͍͜ͱ͕ΒΕ͍ͯΔɻ
§1.2. ૉʹਵ͢Δ༷ʑͳڑͷ֓೦
ೋͭͷ༗ཧa, b∈QͷؒͷʮڑʯΛߟ͑ͨͱ͖ɺڪΒ͘ਅͬઌʹಡऀͷ಄ʹු͔Ϳ ͷɺී௨ͷઈର| − |∞ʹΑΔڑͷఆٛ
|a−b|∞
Ͱͳ͍ͩΖ͏͔ɻ͔͠͠ɺͰɺ͜ͷ| − |∞ Ҏ֎ʹɺૉpʹਵ͢Δʮpਐ ڑʯ| − |pΛߟ͢Δ͜ͱॏཁͰ͋Δɻ͜ͷpਐڑͷఆ͕ٛͩɺ·ͣ|0|p = 0ͱఆٛ
͢Δɻ༗ཧa∈Q͕θϩͰͳ͍ͱԾఆ͢Δͱɺదͳl, n, m∈Zʢͨͩ͠nͱm
= 0͔ͭpͱૉͰ͋ΔͱԾఆ͢Δʣʹରͯ͠ɺa=pl·n·m−1ͱॻ͚Δ͜ͱͪʹ
͔Δ͕ɺͦͷͱ͖ɺ|a|pɹ
|a|p = |pl·n·m−1|p def
= p−l
ͱఆٛ͢Δɻͭ·ΓɺlΛେ͖ͳਖ਼ͷʹ࠾ͬͨͱ͖ɺ|pl|p =p−l ඇৗʹখ͘͞ͳΔͱ
͍͏ɺ௨ৗͷ| − |∞Λجʹͨ͠ײ͔֮Β͢Εɺʮৗࣝ֎Εʯͳݱ͕ى͜Δɻ͜ͷ
l→+∞lim pl = 0 ͱ͍͏ݱͷଞʹɺྫ͑ɺp= 2ͷͱ͖ɺ
−1 = 1
1−2 =
+∞
l=0
2l = 1 + 2 + 22+ 23+ 24+ 25+. . .
ͷΑ͏ʹɺෛͷΛਖ਼ͷͷແݶͱͯ͠දࣔ͢Δ͜ͱ͕Ͱ͖ΔɺpਐڑΛ༻͍
ֶͨͰ௨ৗͷ| − |∞ ͷৗࣝͰߟ͑ΒΕͳ͍Α͏ͳݱ͕ଟݟΒΕΔͷͰ͋Δɻ
࣮ɺ্ड़ͷ| − |∞ ͱ| − |pͷ͍ͣΕಛผͳ߹ʹ֘͢ΔΑ͏ͳͬͱҰൠతͳ
ͷͱͯ͠ʮʯͱ͍͏֓೦͕͋Δɻ͜͜ͰɺҰൠͷͱͲ͏͍͏ͷ͔ͱ͍͏ৄ
͍͠આ໌͠ͳ͍͕ɺ࣮ɺ
༗ཧମ্ͷɺઌఔհͨ͠| − |∞ ͱ| − |p ʹݶΔ
͜ͱ͕ΒΕ͍ͯΔɻ༗ཧମ্ͷΛ༻ޠͰಛఆ͢Δͱ͖ɺ| − |∞ ΞϧΩϝσε
ͱݺͼɺ| − |p ඇΞϧΩϝσεͱݺͿɻɹ
ͱ͍͏֓೦ͷҰൠੑΛ׆༻͢Δͱɺ༗ཧମQ͚ͩͰͳ͘ɺҙͷମF ʹର͠
ͯʮʯΛߟ͢Δ͜ͱ͕ՄೳʹͳΔɻͦͷΑ͏ͳF ͷ্ͷvΛQ ⊆ F ʹ੍ݶ
͢Δͱɺ༗ཧମQ্ͷwɺଈͪʢઌఔհͨ͠ࣄ࣮Λద༻͢Δͱʣ| − |∞ ͔| − |p
͕ग़ͯ͘Δɻ͜ͷ੍ݶʹΑͬͯఆ·Δw͕ΞϧΩϝσεͷͱ͖ɺݩͷF ্ͷ
ΛΞϧΩϝσεͱݺͼɺw͕ඇΞϧΩϝσεͷͱ͖ɺݩͷF ্ͷΛඇΞϧΩ ϝσεͱݺͿɻͳ͓ɺwΛݻఆ͠ɺͦͷwΛൃੜ͢ΔΑ͏ͳvͷू߹Λߟ͑Δͱɺ
wΛൃੜ͢ΔΑ͏ͳF ͷvͷू߹༗ݶͰ͋Γɺ
͔͠ɺͦͷೱ[F :Q]ҎԼͰ͋Δ
͜ͱ͕ΒΕ͍ͯΔɻͭ·Γɺ͜ͷ༗ཧମͷwͷɺF ʹ͓͚Δʮղʯͷ༷ࢠɺ v . . . v
\ | / w
ͷΑ͏ͳʮπϦʔʯ(tree)Λඳ͍͍ͯΔͱ͍͏͜ͱͰ͋Δɻ࣮ɺݹయతͳతͷ
༷ʑͳਂ͍ఆཧʢৄ͘͠[NSW], Theorem 12.2.5 ΛࢀরʣΛద༻͢Δͱɺ
͜ͷͷղͷ༷ࢠʹΑͬͯɺମF ͕ʢຆͲʣܾఆ͞Εͯ͠·͏
͜ͱ͕ΒΕ͍ͯΔɻʮຆͲʯͱɺʮ͋Δൺֱత͍͠ྫ֎త߹Λআ͍ͯʯͱ͍͏ҙຯ Ͱ͋Δɻ
ઌʹਐΉલʹ۩ମྫΛݟͯΈΑ͏ɻମΛF =Q(√
−1)ͱͨ͠ͱ͖ɺ༗ཧମQͷ
wͷF ʹ͓͚Δղͷ༷ࢠʹɺ࣍ͷΑ͏ͳೋछྨ͔͠ͳ͘ɺ
v v
\ /
w
v
| w લऀͷʮܕʯɺͪΐ͏Ͳw =| − |pͰ
p≡1 (mod 4) ͷͱ͖ʹൃੜ͢ΔͷͰ͋Δɻ͜ΕɺF ͷੜݩi=√
−1͕ࠜͱͳΔଟ߲ࣜf(T) =T2+ 1
͕ɺpΛ๏ͱͯ͠ܭࢉͨ͠ͱ͖ɺࠜΛ͔࣋ͭͲ͏͔ͱ͍͏͜ͱͱɺͪΐ͏ͲରԠͯ͠
͍ΔͷͰ͋Δɻྫ͑ɺp= 5Λ๏ͱͯ͠ܭࢉ͢Δͱɺf(T) =T2+ 1±2ͱ͍͏ࠜ
Λ࣋ͭ͜ͱ͕ͪʹ͔Δɻ
ຊߘͷେ͖ͳςʔϚͷҰͭɺମͱʢίϯύΫτͳʣҐ૬ۂ໘ͷྨࣅੑͰ͋Δ͕ɺͦ
ͷྨࣅͰߟ͑Δͱɺ
ମͷݩҐ૬ۂ໘্ͷؔʹରԠ͍ͯͯ͠ɺ
ମͷͨͪͪΐ͏ͲҐ૬ۂ໘ͷʹରԠ͍ͯ͠Δ
ͷͰ͋Δɻͭ·Γɺମͷݩ͕ɺ͋Δʹؔͯ͠େ͖͔ͬͨΓখ͔ͬͨ͞Γ͢Δͱ͍͏ݱ
ɺͪΐ͏ͲҐ૬ۂ໘্ͷ͕ؔɺ͋Δʹۙ͘ʹͭΕɺͲͷҐͷʮ͍ʯͰθϩʹ ऩଋͨ͠Γɺ͋Δ͍ແݶʹൃࢄͨ͠Γ͢Δ͔ɺͱ͍͏͜ͱͱରԠ͍ͯ͠ΔͷͰ͋Δɻ͜
ͷΑ͏ʹମΛزԿతͳରͱͯ͠ଊ͑Δʢਤ̍Λࢀরʣͱɺઌఔհͨ͠ࣄ࣮ɺͭ·Γ ʮͷղͷ༷ࢠʹΑͬͯɺମF ͕ʢຆͲʣܾఆ͞Εͯ͠·͏ʯͱ͍͏ݱ͕ɺΑΓࣗ
વͳڹ͖Λଳͼͯ͘Δͱ͍͏ݟํͰ͖Δɻ
ਤ̍ɿͨͪΛʮʯͱݟ၏͢͜ͱʹΑͬͯɺମΛҰछͷزԿతͳରͱͯ͠ଊ͑Δ
. . . . vʼʼʼ
vʼʼ
vʼ . . . .
. . . . 2
p
3 5 11
7
§1.3. ֦େମͱΨϩΞ܈
͜Ε·Ͱݸผͷମʹର༷ͯ͠ʑͳ֓೦ੑ࣭ʹ͍͖͕ͭͯͯͨ͡ɺຊઅʢ§1.3ʣͰ
ɺแؚؔ
F ⊆ K ⊆ Q
ཱ͕͢ΔΑ͏ͳೋͭͷମF ͱK ʹ͍ͭͯߟ͍ͨ͠ɻ͜ͷΑ͏ͳঢ়گͷͱ͖ɺK Λ F ͷ֦େମͱݺͼɺͦͷʮ֦େ࣍ʯ
[K :F] def= [K :Q]/[F :Q]
͕ʹͳΔ͜ͱ؆୯ʹূ໌Ͱ͖Δɻ §1.1 Ͱɺݸผͷମʹରͯ͠ʮʢQ্ͰʣΨϩ ΞͰ͋Δʯͱ͍͏ੑ࣭ʹ͍ͭͯߟ͕͑ͨɺͦͷఆٛͷதͰʮ༗ཧͷଟ߲ࣜʯΛʮF
ͷଟ߲ࣜʯʹஔ͖͑Δ͜ͱʹΑͬͯɺʮK ͕F ্ͰΨϩΞͰ͋Δʯͱ͍͏ੑ࣭Λఆ
ٛ͢Δ͜ͱ͕Ͱ͖Δɻ·ͨɺಉ༷ʹͯ͠ʮମKͷF ্ͷੜݩʯͱ͍͏֓೦Λఆٛ͢Δ
͜ͱ͕ՄೳͰ͋Δɻ
ҎԼͷٞͰɺK ͕F ্ͰΨϩΞͰ͋ΔͱԾఆ͠Α͏ɻͦ͏͢ΔͱɺͷதͰ
த৺తͳ֓೦ͷҰͭͰ͋ΔʮK ͷF ্ͷΨϩΞ܈ʯ Gal(K/F)
Λఆٛ͢Δ͜ͱ͕Ͱ͖Δɻ͜ͷू߹Gal(K/F)ͷݩσɺՃݮআʹʮମͷߏʯͱཱ྆
తͳK ͔ΒKͷࣸ૾
σ : K → K
ͰɺF ⊆Kʹ੍ݶ͢Δͱʮ߃ࣸ૾ʯʹͳΔʢʹͭ·Γɺҙͷx∈Fʹରͯ͠ɺσ(x) =x
ཱ͕͢ΔʣͷͰ͋Δɻࣸ૾Gal(K/F)σ :K →Kɺ࣮ɺ
K ͷF ্ͷੜݩͨͪͷߦઌ͚ͩͰશʹܾఆ͞Εͯ͠·͏
͜ͱ؆୯ʹূ໌Ͱ͖Δɻͳ͓ɺ͜ͷΑ͏ʹఆٛͨ͠ू߹Gal(K/F)ɺ
༗ݶू߹ʹͳΓɺ͔ͦ͠ͷೱͪΐ͏Ͳ֦େ࣍[K :F]ͱҰக͢Δ
͜ͱ؆୯ʹূ໌Ͱ͖Δɻ໊শ͔Βਪଌ͞ΕΔ௨ΓɺGal(K/F)ͱ͍͏ू߹ʹࣗવͳʮ܈
ߏʯ͕ೖΔɻҰൠʹू߹Gͷ্ͷʮ܈ߏʯͱɺ݁߹๏ଇΛຬͨ͢ʮ܈ԋࢉʯΛఆΊ Δࣸ૾
G×G → G
(g, h) → g·h
Ͱɺߋʹɺ୯Ґݩe ∈Gͷଘࡏʢʹͭ·Γɺg·e=e·g =g,∀g ∈Gʣͱٯݩͷଘࡏʢʹͭ
·Γɺ∀g ∈G, g·h = h·g= eΛຬͨ͢h ∈GͷଘࡏʣΛԾఆ͢ΔɻGal(K/F)ͷ߹ɺ σ, τ ∈Gal(K/F)ʹରͯ͠ɺࣸ૾ͷ߹
σ·τ : K −→τ K −→σ K ʹΑͬͯ܈ߏΛೖΕΔͷͰ͋Δɻ
࣍ʹ۩ମྫΛز͔ͭݟͯΈΑ͏ɻ·ͣɺQ(√
−1) =Q(i)͕ͩɺෳૉڞࣸ૾Λ σ : Q(i) → Q(i)
i → −i
ͱද͢ͱɺʢ༰қʹ֬ೝͰ͖ΔΑ͏ʹʣ
Gal(Q(i)/Q) = {id, σ} ʢͨͩ͠ɺid߃ࣸ૾ʣͱͳΔɻ࣍ʹɺK def= Q(√3
2,√
−3) = Q(√3
2, ω); F def= Q(ω) ͱஔ͘ͱɺK ͕F ্ͰΨϩΞͰ͋Δ͜ͱͪʹ֬ೝͰ͖ɺ
σ : K → K τ : K → K
√3
2 → √3
2 √3
2 → ω·√3 2
ω → ω2 ω → ω
ͱఆΊΔͱɺ
Gal(K/Q) = {id, σ, τ, τ·σ, τ2, τ2·σ} Gal(K/F) = {id, τ, τ2}
ʢͨͩ͠ɺid߃ࣸ૾ʣͱͳΔ͜ͱ؆୯ͳܭࢉʹΑͬͯ֬ೝ͢Δ͜ͱ͕Ͱ͖Δɻ
͜Ε·Ͱݟ͖ͯͨΑ͏ͳ༗ݶͳΨϩΞ܈Gal(K/F)ͷଞʹɺແݶͳΨϩΞ܈Gal(Q/F) Λఆٛ͢Δ͜ͱՄೳͰ͋Δɻͭ·Γɺू߹Gal(Q/F)ͷݩσɺՃݮআʹʮମͷߏʯ ͱཱ྆తͳQ͔ΒQͷࣸ૾
σ : Q → Q
ͰɺF ⊆Qʹ੍ݶ͢Δͱʮ߃ࣸ૾ʯʹͳΔͷͰ͋Δɻ·ͨɺ༗ݶͳΨϩΞ܈ͷͱ͖ͱ ಉ༷ʹɺࣸ૾ͷ߹Λߟ͑Δ͜ͱʹΑͬͯɺू߹Gal(Q/F)ʹࣗવͳ܈ߏΛೖΕΔ͜ͱ
͕Ͱ͖Δɻ͜ͷΑ͏ʹఆٛ͢Δͱɺࣸ૾Gal(Q/F) σ : Q → QΛK ʹ੍ݶ͢Δ͜ͱ ʹΑͬͯɺࣗવͳ४ಉܕʢʹఆٛҬͱҬͷͦΕͧΕͷ܈ԋࢉͱཱ྆తͳࣸ૾ʣ
Gal(Q/F) Gal(K/F)
Λఆٛ͢Δ͜ͱ͕Ͱ͖Δɻ͜ͷ४ಉܕʢʮʯͱ͍͏ه߸͕͍ࣔͯ͠ΔΑ͏ʹʣશࣹʢʹ
Ҭͷͯ͢ͷݩ͕૾ʹೖΔʣʹͳΔͨΊɺGal(K/F)ΛɺGal(Q/F)ͷ(quotient)ɺͭ
·ΓɺԿΒ͔ͷҙຯʹ͓͍ͯGal(Q/F)ΛʮͿͬͭͿ͢ʯ͜ͱʹΑͬͯߏ͞Εͨͷͱ ݟΔ͜ͱ͕Ͱ͖ΔɻٯʹɺF ͷʢΨϩΞʣ֦େମK Λಈ͔͢͜ͱʹΑͬͯɺGal(Q/F)Λɺ Ұछͷʮ܈ͷۃݶʯ=ʮٯۃݶʯ
Gal(Q/F) = lim←−K Gal(K/F)
ͱࢥ͏͜ͱ͕Ͱ͖ΔɻҰํɺK ͱF ͱ͍͏ʮରʯʹఆ͕ٛຊ࣭తʹґଘ͢Δ༗ݶͳΨϩΞ
܈Gal(K/F)ͱҧͬͯɺGF def= Gal(Q/F)ͱ͍͏܈ɺʢࣄ্࣮ʣମF ͚ͩͰఆٛͰ͖
ΔͷͰ͋Δɻଈͪɺɹ
GF ͱ͍͏܈ɺମF ʹਵ͢Δʮෆมྔʯ
ͱݟΔ͜ͱ͕Ͱ͖Δɻ͜ͷʮෆมྔʯGF ɺͦͷॏཁੑ͔ΒF ͷʮઈରΨϩΞ܈ʯͱݺ
ΕɺͰத৺తͳݚڀରͷҰͭͱͳ͍ͬͯΔɻͨͩɺGF Λఆٛ͢ΔͨΊʹɺ ಛఆͷ֦େମʮKʯΛࢦఆ͢Δඞཁ͕ͳͯ͘ɺ
֓೦ͷ্ʹ͓͍ͯʮ֦େମʯͦͷʮΨϩΞ܈ʯ͕ඞཁෆՄܽͰ͋Δ
͜ͱΛΕͯͳΒͳ͍ɻ࠷ޙʹɺ͜Ε·ͰͷۃΊͯॳతͳΨϩΞ܈ͷͱҧͬͯʢNeukirch-
ాͷʣ͍͠ఆཧʢৄ͘͠[NSW], Theorem 12.2.1 ΛࢀরʣͰ͋Δ͕ɺ࣮ɺಉܕ Λআ͍ͯߟ͑Δͱɺ
ମF ɺͦͷઈରΨϩΞ܈GF ʹΑͬͯશʹܾఆ͞ΕΔ ͷͰ͋Δɻ
§2. Ґ૬ۂ໘্ͷྠମͱඃ෴
§2.1. ίϯύΫτͳҐ૬ۂ໘ͷఆٛͱछ
ຊઅʢ§2.1ʣ͔ΒҐ૬ۂ໘ͷزԿʹ͍ͭͯߟ͢Δɻ·ͣఆ͔ٛΒ࢝Ί͍͕ͨɺʮҐ૬ ۂ໘ʯͱɺॳతͳݴ༿Ͱ͍͏ͱɺہॴతʹ୯Ґԁ൫ͱಉܕͳʮزԿֶతରʯʢʹਖ਼֬
ʹ͍͏ͱɺҐ૬ۭؒʣͷ͜ͱͰ͋Δʢਤ̎Λࢀরʣɻผͷݴ͍ํΛ͢Δͱɺ
ࢁͷ୯Ґԁ൫ͷίϐʔΛ࿈ଓʹషΓ߹ΘͤͯͰ͖ͨزԿֶతରͰ͋Δɻ ຊߘͰɺײతͳݴ༿Ͱ͍͏ͱɺجຊతʹແݶʹ͕ΔΑ͏ͳҐ૬ۂ໘Ͱͳ͘ɺԿ Β͔ͷҙຯʹ͓͍ͯʮ༗ݶͳʹ༗քͳ͕Γʯ͔࣋ͨ͠ͳ͍Α͏ͳҐ૬ۂ໘Λத৺ʹΛ ਐΊ͍ͨɻֶ༻ޠʹ༁͢Δͱɺ͜ΕɺʮίϯύΫτʯͳҐ૬ۂ໘ʹΛݶఆ͢Δͱ͍
͏͜ͱͰ͋ΔɻʮίϯύΫτʯͳҐ૬ۂ໘S ͱɺແݶͳྻ
s1, s2, . . . , sn, . . . ∈S
͕ඞͣʢগͳ͘ͱ̍ͭͷʣूੵΛ࣋ͭΑ͏ͳҐ૬ۂ໘ͷ͜ͱͰ͋Δɻݴ͍͑Εɺ্
هͷྻͷదͳ෦ྻt1, t2, . . . , tn, . . .∈S Λ࠾Εɺۃݶ
nlim→∞ tn = t
͕tʹऩଋΑ͏ͳt∈S͕ଘࡏ͢Δͱ͍͏͜ͱͰ͋Δɻ
ਤ̎ɿ୯Ґԁ൫ͱہॴతʹಉܕͳҐ૬ۂ໘
ຊߘͰऔΓѻ͏Ґ૬ۂ໘ʹରͯ͠ɺίϯύΫτੑͷଞʹ͏Ұͭͷٕज़తͳ݅Λ
՝͍ͨ͠ɻͦΕɺʮ͖͚ՄೳͰ͋Δʯͱ͍͏݅Ͱ͋Δɻʮ͖͚Մೳʯ(orientable) ͳҐ૬ۂ໘ɺʮ؍ऀʯ͕ۂ໘ͷͲͷʹཱ͍ͬͯͯɺͦͷ͔Βݟͯͷʮ࣌ܭճΓʯ ʢ͋Δ͍ʮ࣌ܭճΓʯʣͱ͍͏֓೦ΛɺΛ࿈ଓʹಈ͔ͨ͠ͱ͖ໃ६Λདྷ͢͜ͱͳ͘ఆ
ٛͰ͖Δۂ໘ͷ͜ͱͰ͋Δɻ͜Εٕज़తͳ͕݅ͩɺɾزԿؔͰɺগ ͳ͘ͱ௨ৗͷઃఆͰɺࣗવʹൃੜ͢ΔҐ૬ۂ໘ඞ͖͚ͣՄೳʹͳΔͨΊɺ͜ͷ
݅ʹ͍ͭͯຊߘͰ͜ΕҎ্͡ͳ͍͜ͱʹ͢Δɻ؆୯ͷͨΊɺҎԼͰʮҐ૬ۂ໘ʯ ͱॻ͍ͨͱ͖ɺ͖͚ՄೳͳҐ૬ۂ໘Λҙຯ͢Δͱ͍͏͜ͱʹ͢Δɻ
Ґ૬زԿֶͷ͔ͳΓݹయతͳఆཧʹͳΔ͕ɺ࣮ɺίϯύΫτ͔ͭ࿈݁ʢʹڞ௨෦͕
ۭʹͳΔΑ͏ͳೋͭͷۭͰͳ͍։ू߹ͷू߹ͱͯ͠දࣔ͢Δ͜ͱ͕Ͱ͖ͳ͍ʣͳҐ૬ۂ ໘ʮछʯʢ∈NʣͱݺΕΔෆมྔʹΑͬͯશʹྨ͞Ε͍ͯΔɻͭ·ΓɺಉܕΛআ
͍ͯߟ͑ΔͱɺʢίϯύΫτ͔ͭ࿈݁ͳʣ
Ґ૬ۂ໘ͦͷछʹΑͬͯશʹܾఆ͞ΕΔ
ͱ͍͏͜ͱͰ͋Δɻछ0ͷʢίϯύΫτ͔ͭ࿈݁ͳʣҐ૬ۂ໘ʢॳతزԿͰ͓ೃછ Έͷʣٿ໘Ͱ͋Δʢਤ̏Λࢀরʣɻछg≥1ͷʢίϯύΫτ͔ͭ࿈݁ͳʣҐ૬ۂ໘ͷྫ
࣍ͷΑ͏ʹؼೲతʹߏ͢Δ͜ͱ͕Ͱ͖Δɿɹ ʢΞʣछg−1ͷۂ໘ʹೋͭͷ݀Λ։͚ɺ ʢΠʣʢผͷʣछ0ͷۂ໘ʹೋͭͷ݀Λ։͚ɺ
ʢʣʢΞʣͱʢΠʣͷۂ໘ΛɺͦΕͧΕͷ݀ͷʹԊͬͯ๓͍߹ΘͤΔɻ
ࡉ͔͍͕ͩɺ࣮ʮ๓͍߹ΘͤΔʯͱ͖ɺͦΕͧΕͷۂ໘ͷʮ͖͚ʯͱཱ྆తͳܗ Ͱ๓߹͠ͳ͍ͱ͍͚ͳ͍͕ɺຊߘͰ͜ͷΑ͏ͳٕज़తͳʹؔ͢Δৄ͍͠આ໌লུ͢
Δɻʮదʯͳٕज़తͳ݅ͷԼͰ๓߹Λ࣮ߦ͢Δͱɺਤ̏Ͱࣔͨ͠Α͏ʹɺछ1ͷ߹
ʹυʔφπͷද໘ͷΑ͏ͳܗΛͨ͠ۂ໘͕ग़དྷ্͕Γɺछgͷ߹ʹgݸͷυʔφ πͷද໘Λचܨ͗ʹషΓ߹ΘͤͨΑ͏ͳܗΛͨ͠ۂ໘͕ग़དྷ্͕Δɻ
ਤ̏ɿछ0ʢʹٿ໘ʣ, 1, 2 ͷʢίϯύΫτͰ͖͚ՄೳͳʣҐ૬ۂ໘ ɹ
§2.2. Ґ૬ۂ໘ͷجຊ܈
Ґ૬ۂ໘ͷزԿΛௐΔ্ʹ͓͍ͯ࠷ॏཁͳಓ۩ͷҰͭجຊ܈Ͱ͋ΔɻʢίϯύΫτ
͔ͭ࿈݁ͳʣҐ૬ۂ໘Sͷجຊ܈Λఆٛ͢ΔͨΊʹɺ·ͣSͷs∈S ΛҰͭબΜͰݻ ఆ͢Δඞཁ͕͋Δɻ͢ΔͱɺsΛج(basepoint)ͱ͢ΔS ͷجຊ܈(fundamental group)
π1(S, s)
ͷԼ෦ू߹ɺ࢝ऴs ʹͳΔΑ͏ͳด࿏ʹดಓ(closed path)Λɺʮ࿈ଓͳมܗʯ ʢʹֶ༻ޠͰ͍͏ͱɺϗϞτϐʔ(homotopy)ʣΛআ͍ͯߟ͑Δ͜ͱʹΑͬͯͰ͖Δಉ
ྨͷू߹ͱͯ͠ఆٛ͢ΔɻͦͷԼ෦ू߹ͷݩα, βʹରͯ͠ɺ࿏ͷ߹
α◦β
ʢͭ·Γɺ࿏βʹԊͬͯҠಈ͔ͯ͠Βɺ࿏αʹԊͬͯҠಈ͢Δ͜ͱʹΑͬͯͰ͖Δ࿏ʣΛର Ԡͤ͞Δ͜ͱʹΑͬͯπ1(S, s)্ʹ܈ߏΛೖΕΔɻ͜Ε͕جຊ܈ͷʢ܈ͱͯ͠ͷʣఆٛ
Ͱ͋Δɻ
ҰൠʹɺG͕܈ͩͱ͢ΔͱɺGͷ܈ߏͱཱ྆తͳG͔ΒGͷࣸ૾
σ: G →∼ G
Ͱશ୯ࣹʹͳΔͷͷ͜ͱΛ܈Gͷࣗݾಉܕ(automorphism)ͱݺͼɺGͷࣗݾಉܕશମΛ Aut(G)
ͱ͍͏ه߸Ͱද͢ɻࣗݾಉܕͷ߹Λߟ͑Δ͜ͱʹΑͬͯɺAut(G)ͱ͍͏ू߹ʹࣗવͳ
܈ߏ͕ೖΔɻ܈Gͷݩh∈GΛҰͭ࠾ΔͱɺhʹΑΔʮڞʯ(conjugation) G g → h·g·h−1 ∈ G
ʹΑͬͯɺAut(G)ͷݩγh ∈Aut(G)͕Ұͭఆ·ΔΘ͚͕ͩɺ͜ͷΑ͏ͳࣗݾಉܕγh ͱ͠
ͯੜ͡ΔAut(G)ͷݩͷ͜ͱΛ෦ࣗݾಉܕͱݺͼɺGͷ෦ࣗݾಉܕશମΛ
Inn(G) ⊆ Aut(G)
ͱ͍͏ه߸Ͱද͢ɻ·ͨɺσ, τ ∈ Aut(G)ʹରͯ͠ɺσ = τ ·γ ͱͳΔΑ͏ͳ෦ࣗݾಉܕ γ ∈ Inn(G)͕ଘࡏ͢Δͱ͖ɺσͱτ ɺGͷಉҰͷ֎෦ࣗݾಉܕ (outer automorphism) ΛఆΊΔͱ͍͏ݴ͍ํΛ͢ΔɻGͷ֎෦ࣗݾಉܕશମ
Out(G)
ͱ͍͏ه߸Ͱද͢ɻ͜ͷOut(G)ͱ͍͏ू߹ʹɺ֎෦ࣗݾಉܕͷ߹ʹΑΓࣗવͳ܈ߏ
͕ೖΓɺ·ͨҙͷࣗݾಉܕʹରͯͦ͠ΕʹΑͬͯఆ·Δ֎෦ࣗݾಉܕʢʹࣗݾಉܕͷ ҰछͷಉྨʣΛରԠͤ͞Δ͜ͱʹΑͬͯʢInn(G)Λʮ֩(kernel)ʯͱ͢ΔΑ͏ͳʣࣗવ ͳ४ಉܕ
Aut(G) Out(G)
͕ఆ·Δɻ
ͯ͞Sͷجຊ܈ͷʹΖ͏ɻπ1(S, s)ͷఆٛجsͷબʹґଘ͢ΔΘ͚͕ͩɺ࣮ɺ
෦ࣗݾಉܕΛআ͍ͯߟ͑ΔΑ͏ʹ͢Δͱɺπ1(S, s)جsͷબʹґଘ͠ͳ͍
͜ͱ؆୯ʹࣔͤΔɻैͬͯɺ෦ࣗݾಉܕΛແࢹͯ͠࡞ۀͯ͠ߏΘͳ͍Α͏ͳɺଟ͘
ͷઃఆͰɺجͷಛఆʹݴٴ͠ͳ͔ͬͨΓɺه߸ͷ্ʹ͓͍ͯ
π1(S) ͱॻ͍ͨΓ͢Δ͜ͱ͕͋Δɻ
࠷ޙʹɺπ1(S)ͱ͍͏܈ͷߏʹ͍ͭͯ͏গ͠ৄ͘͠આ໌͍ͨ͠ɻఆ্ٛɺS্ͷ
ҙͷด࿏ʢجͷΛແࢹ͢Εʣπ1(S)ͷݩΛఆΊΔ͕ɺࣗࣗͱަΘͬͨΓ͠ͳ
͍ɺʮదʯͳҙຯʹ͓͍ͯʮ͖Ε͍ʯͳྠମʢਤ̐ΛࢀরʣΛ࠾Δ͜ͱʹΑͬͯɺπ1(S) ͱ͍͏܈ͷੜݩͷܥ(system of generators)
α1, β1, α2, β2, . . . αg, βg
ʢͨͩ͠ɺgSͷछΛද͢ͱ͢ΔʣΛߏ͢Δ͜ͱ͕Ͱ͖Δɻ͜ͷΑ͏ͳʮ͖Ε͍ʯͳ ੜݩͷܥͩͱɺҰͭͷʢൺֱత؆໌ͳʣؔࣜ
α1·β1·α−11 ·β1−1·α2·β2·α−12 ·β2−1·. . .·αg·βg·α−1g ·βg−1 = 1
͚ͩͰπ1(S)ͷ܈ͱͯ͠ͷߏ͕શʹܾ·Δ͜ͱ͕ΒΕ͍ͯΔɻ
ਤ̐ɿҐ૬ۂ໘্ͷදతͳྠମ
§2.3. Ґ૬ۂ໘ͷඃ෴ͱඃ෴ม܈
લઅʢ§2.2ʣͰɺด࿏ʹΑΔجຊ܈ͷఆٛʹ͍ͭͯղઆ͕ͨ͠ɺͦͷఆٛͩͱɺجຊ
܈ͱͷਂ͍͕ؔͲͷΑ͏ʹͯ͠ੜ͡Δ͔ɺগͳ͘ͱతʹઆ໌͢Δ͜ͱ͔
ͳΓࠔͰ͋ΔɻҰํɺຊઅʢ§2.3ʣͰղઆ͢Δඃ෴ʹΑΔجຊ܈ͷఆٛΛ༻͍Δͱɺඇৗ
ʹಁ໌ͳܗͰ
جຊ܈ͱʢʹ۩ମతʹɺ §1Ͱհͨ͠ΨϩΞ܈ͷཧʣΛؔ࿈͚Δ
͜ͱ͕ՄೳʹͳΔɻ
·ͣɺඃ෴ͷఆ͔ٛΒ࢝ΊΑ͏ɻS͕ʢίϯύΫτͱݶΒͳ͍ʣ࿈݁ͳҐ૬ۂ໘ͩͱ
͢ΔͱɺSͷඃ෴
f : T → S
ͱɺҐ૬ۂ໘T ͔ΒSͷ࿈ଓࣸ૾Ͱɺ࣍ͷʢʮہॴࣗ໌ੑʯͷʣ݅Λຬͨ͢ͷͰ
͋ΔɿSͷҙͷs ∈Sͱͦͷsͷेʹখ͍͞։ۙU ʹରͯ͠ɺU ⊆Sͷ੍ݶ ʹΑͬͯಘΒΕΔ࿈ଓࣸ૾ɹ
f|U : T|U → U
U ͷʢز͔ͭͷʣίϐʔͷʹΑͬͯఆ·Δʮࣗ໌ͳඃ෴ʯ U → U
ͱಉܕʹͳΔɻU ͷίϐʔͷݸ͕༗ݶͳͱ͖ɺඃ෴Λʮ༗ݶ࣍ඃ෴ʯͱݺͼɺແݶͳͱ
͖ɺඃ෴Λʮແݶ࣍ඃ෴ʯͱݺͿɻ·ͨɺඃ෴T →Sʹରͯ͠ɺͦΕʹਵ͢Δʮඃ෴
ม܈ʯ
Aut(T /S)
࣍ͷΑ͏ʹఆٛ͞ΕΔɿAut(T /S)ͷݩσ
σ : T → T
ɺఆٛҬͱҬͷͦΕͧΕͷT ͷҐ૬ͱཱ྆తͳશ୯ࣹͰɺf =f◦σͱ͍͏݅Λຬͨ
͢ͷͰ͋Δɻ͜ͷఆٛɺʢ§1.3 Ͱղઆͨ͠ʣ
ΨϩΞ܈ͷఆٛͱܗࣜతʹΑ͘ࣅ͍ͯΔ
͕ɺ࣮ࡍɺT ͕࿈͔݁ͭ࣍ͷ݅Λຬͨ͢ͱ͖ɺඃ෴f :T → SΛʮΨϩΞͳඃ෴ʯͱݺ ͿͷͰ͋Δɿ্ड़ͷඃ෴ͷఆٛʹग़͖ͯͨU ͷίϐʔͷ
U ʹରͯ͠Aut(T /S)Λ
࡞༻ͤͨ͞ͱ͖ɺίϐʔͨͪͷ࡞༻͕ਪҠత (transitive)ʹͳΔɺͭ·Γɺҙͷೋͭͷ ίϐʔ“U” ͱ “U” ʹରͯ͠ɺU ΛU ʹࣸ͢
U ⊇ U →∼ U ⊆ U Α͏ͳσ ∈Aut(T /S)͕ඞͣଘࡏ͢Δɻɹ
ҰൠͷSͷ্Ͱ༷ʑͳ࿈݁ͳඃ෴͕͋ΒΘΕΔ͜ͱ͕͋Δ͕ɺͦͷதͰಛච͖͢
ͷͱͯ͠ɺʮීวඃ෴ʯͱݺΕΔಛผ͔ͭʢಉܕΛআ͍ͯʣҰҙʹܾ·Δඃ෴͕͋Δɻ
ීวඃ෴ɹ
S → S
͕ຬͨ͢ಛผͳੑ࣭࣍ͷ௨ΓͰ͋Δɿҙͷ࿈݁ͳඃ෴T →Sɺීวඃ෴S→S ͷ தؒඃ෴ͱͯ͠ੜ͡Δɺͭ·ΓɺS→Sɺ
S → T → S
ͷΑ͏ͳܗͷ߹ࣸ૾ͱͯ͠දࣔ͢Δ͜ͱ͕Ͱ͖Δʢਤ̑Λࢀরʣɻ͜ͷͱ͖ɺT ͕ΨϩΞ Ͱ͋ΔͱԾఆ͢Δͱɺ͜ͷதؒඃ෴ͱͯ͠ͷදࣔʹΑΓɺͦΕͧΕͷඃ෴ม܈ͷؒʹશ
ࣹͳ४ಉܕɹ
Aut(S/S) Aut(T /S)
͕Ҿ͖ى͜͞ΕΔɻ࣮ɺ෦ࣗݾಉܕΛআ͍ͯߟ͑ΔΑ͏ʹ͢Δͱɺ
ීวඃ෴ͷඃ෴ม܈Aut(S/S) جຊ܈π1(S)ͱࣗવʹಉܕʹͳΔɻ
͜ͷࣗવͳಉܕ
Aut(S/S) →∼ π1(S)
࣍ͷΑ͏ʹఆٛ͞ΕΔʢਤ̑Λࢀরʣɿπ1(S)ͷݩΛఆΊΔS ্ͷด࿏α ͕༩͑ΒΕΔ ͱɺαɺS্ͷʢҰൠʹด͍ͯ͡ͳ͍ʂʣ࿏αʹ্͕࣋ͪΔ͕ɺͦͷαͷ࢝Λऴ
ʹࣸ͢Α͏ͳσ ∈Aut(S/S)ʢɿσ ͷଘࡏ্ड़ͷਪҠੑΑΓͪʹै͏ʂʣɺ্هͷ
ࣗવͳಉܕʹΑͬͯαʹରԠ͍ͯ͠Δ
σ → α ͷͰ͋Δɻ
ਤ̑ɿҐ૬ۂ໘ͷ༗ݶ࣍ඃ෴ͱͦͷ্ʹ͋Δʢແݶ࣍ͷʣීวඃ෴
ීวඃ෴S→S ɺҰൠʹʢྫ͑S ͕ʢίϯύΫτ͔ͭ࿈݁ͳʣछg ≥1Ґ૬ ۂ໘ͷͱ͖ʣɺແݶ࣍ඃ෴ʹͳΔɻҰํɺزԿతͳઃఆͰɺ࣮ɺ༗ݶ࣍ඃ෴͔͠
ѻ͏͜ͱ͕Ͱ͖ͳ͍ɻैͬͯɺزԿతͳઃఆͰɺҐ૬زԿతͳઃఆͰ༻͍ΒΕΔج ຊ܈π1(S)ඃ෴ม܈Aut(S/S) ͷΘΓʹɺͦͷ෭༗ݶඋԽͱݺΕΔ࣍ͷΑ͏ͳ ʮٯۃݶʯʢʹ §1.3ʹग़͖ͯͨʮٯۃݶʯͷΛࢀরʂʣ
π1(S) def= Qlim←−
π1
Qπ1 →∼ Qlim←−
Aut
QAut
ʢͨͩ͠ɺQπ1 ܈ π1(S) ͷ༗ݶͳ π1(S) Qπ1 ΛɺQAut ܈Aut(S/S) ͷ༗ݶͳ
π1(S) Qπ1 ΛΔͱ͢ΔʣΛ༻͍Δ͜ͱ͕ଟ͍ɻ͜ͷΑ͏ʹఆٛ͞Εͨ π1(S)ɺ S ͷ෭༗ݶجຊ܈ͱݺͿɻઌఔͷதؒඃ෴ͷͰߟ͑ΔͱɺAut(S/S) ͷ߹ɺ༗ݶͳ
Aut(S/S) QAut ͪΐ͏ͲT ͕༗ݶ࣍ඃ෴ʹͳΔ߹ʹରԠ͍ͯ͠Δɻ
§3. ίϗϞϩδʔʹΑΔʮ࣍ݩʯͷఆٛ
§3.1. Ґ૬زԿʹ͓͚ΔίϗϞϩδʔ࣍ݩ
ֶͰɺʮۭؒͷ࣍ݩʯͱ͍͏ײతͳ֓೦ʹର༷ͯ͠ʑͳఆࣜԽͷํ͕͋Δ͕ɺҐ ૬زԿֶͰɺίϗϞϩδʔՃ܈ʹΑΔख๏͕࠷جຊతͳΞϓϩʔνͰ͋ΔɻҰൠʹɺʢద
ͳٕज़త݅Λຬͨ͢ʣҐ૬ۭؒX ͱࣗવnʹରͯ͠ɺn࣍ίϗϞϩδʔՃ܈(n-th cohomology module)
Hn(X)
ͱݺΕΔՃ܈(module)ɺͭ·ΓɺՄͳ܈ԋࢉΛ࣋ͭ܈ΛରԠͤ͞Δ͜ͱ͕Ͱ͖Δɻຊ ߘͰɺ͜ͷHn(X)ͷݫີͳఆٛͷৄ͍͠આ໌লུ͢Δ͕ɺදతͳྫΛڍ͛ΔͱɺX ͱͯ͠m࣍ݩٿ໘
Sm = {(x1, x2, . . . , xm+1)∈Rm+1 | x21+x22+. . .+x2m+1 = 1} ⊆ Rm+1 Λ࠾༻͢ΔͱɺͦͷίϗϞϩδʔՃ܈࣍ͷΑ͏ʹͳΔɿ
Hn(Sm) = Z ∀ n∈ {0, m} Hn(Sm) = {0} ∀ n∈ {0, m}
ͭ·Γɺ͘͝ࡶͳݴ͍ํΛ͢Δͱɺ
n࣍ίϗϞϩδʔՃ܈ͪΐ͏ͲۭؒXͷதʹʮn ࣍ݩͷ݀ʯ͕ͲͷҐ͋Δ͔
Λଌ͍ͬͯΔͷͱݟΔ͜ͱ͕Ͱ͖Δɻ͜ͷΑ͏ͳྫΛ౿·͑ͯߟ͑ΔͱɺҰൠͷʢྫ͑
ɺ؆୯ͷͨΊɺίϯύΫτ͔ͭ࿈݁ͳʣXͷ߹ɺͦͷ࣍ݩdΛɺɹ Hd(X)={0}; Hn(X) ={0} ∀ n > d Λຬͨ͢Α͏ͳdͱͯ͠ఆٛ͢Δ͜ͱࢸͬͯࣗવͰ͋Δɻ
Ͱɺ§2 Ͱߟͨ͠ʢίϯύΫτ͔ͭ࿈݁ͳʣछgҐ૬ۂ໘Sͷ߹ʹઌఔͷٞ
Λద༻͢ΔͱͲ͏ͳΔ͔ɺߟ͑ͯΈ͍ͨɻ·ͣɺg= 0ͷ߹ɺSʢ2࣍ݩʣٿ໘ʹͳΔ
ͨΊɺઌఔͷٞΛద༻͢Δͱɺͦͷʮ࣍ݩʯ͔֬ʹʢײ௨Γͷʣ2ʹͳΔɻҰํɺछ
g≥1ͷSͩͱɺ §2.1 Ͱݟ͖ͯͨΑ͏ʹɺʢෳͷʣٿ໘͔Βग़ൃ༷ͯ͠ʑͳʮషΓ߹
Θͤʯͷૢ࡞ʹΑͬͯߏ͢Δ͜ͱ͕Ͱ͖Δɻࡉ͔͘ܭࢉ͢Δͱɺ͜ͷషΓ߹Θͤͷૢ࡞
ʹΑͬͯɺʮ1࣍ݩͷ݀ʯ͕ࢁՃ͞ΕΔ͜ͱʹͳΔ͕ɺʮ2࣍ݩͷ݀ʯɺٿ໘ͷ߹
ͱશ͘มΘΒͳ͍ɻͳ͓ɺn≥3ʹରͯ͠ɺʮn࣍ݩͷ݀ʯΛՃ͢ΔΑ͏ͳૢ࡞શ͘ݟ
ͨΒͳ͍͜ͱʹ͍ͨ͠ɻͭ·Γɺઌఔհͨ͠ίϗϞϩδʔՃ܈ʹΑΔ࣍ݩͷఆٛ
Λద༻͢Δͱɺҙͷࣗવg≥0ʹରͯ͠ɺ
छgҐ૬ۂ໘ͷ࣍ݩɺ2ʹͳΔ
͜ͱ͕ؼ݁͞ΕΔɻ
§3.2. Ґ૬ۂ໘ͷίϗϞϩδʔ࣍ݩ
લઅʢ§3.1ʣͰɺίϗϞϩδʔՃ܈ʢҐ૬ʣۭؒX ʹରͯ͠ରԠͤ͞ΒΕΔͷͱ
ͯ͠հ͕ͨ͠ɺ࣮ɺదͳزԿత݅Λຬۭͨؒ͢X ͷ߹ɺίϗϞϩδʔՃ܈
X ͷجຊ܈π1(X)ͱ͍͏நతͳ܈ͷΈʹΑͬͯఆ·Δͷͱͯ͠ఆٛ͢Δ͜ͱՄೳ
Ͱ͋Δɻ͜ͷΑ͏ͳఆٛΛ༻͍Δͱʮn࣍܈ίϗϞϩδʔՃ܈ʯ(n-th group cohomology module) Hn(π1(X))ͱݺΕΔͷ͕ग़དྷ্͕ΓɺʢX͕దͳزԿత݅Λຬͨ͢ͱ͖ʣ
Hn(π1(X)) →∼ Hn(X)
ͷΑ͏ͳܗͷࣗવͳಉܕఆ·ΔɻલઅͰɺ௨ৗͷίϗϞϩδʔՃ܈͕ͲͷൣғͰফ͑
Δ͔ΛݟΔ͜ͱʹΑͬͯɺۭؒXͷʮ࣍ݩʯΛఆٛ͢Δ͜ͱ͕Ͱ͖͕ͨɺX͕దͳزԿ త݅Λຬͨ͢ͱ͖ɺ্ड़ͷٞΛ౿·͑ͯߟ͑Δͱɺ
Xͷجຊ܈π1(X)ͷΈʹΑͬͯఆ·Δʮ࣍ݩʯΛఆٛ͢Δ͜ͱ͕Մೳ
Ͱ͋Δ͜ͱ͕͔Δɻ
લઅͷٞͰɺn࣍ίϗϞϩδʔՃ܈Hn(X)ʹରͯ͠ɺʮn࣍ݩͷ݀ʯʹΑΔղऍʹ
͍ͭͯઆ໌͕ͨ͠ɺ܈ίϗϞϩδʔͷ߹ɺͦ͜·Ͱײత͔ͭزԿతͳղऍ͕Ͱ͖ͳ͘
ͯɺۃΊͯࡶͳϨϕϧͰߟ͢Δͱɺ
1࣍܈ίϗϞϩδʔՃ܈H1(G)ɺ܈GͷੜݩͨͪΛநग़͍ͯ͠Δͷ ͱݟΔ͜ͱ͕Ͱ͖Δͷʹରͯ͠ɺ
2࣍܈ίϗϞϩδʔՃ܈H2(G)ɺ
ͦͷੜݩͨͪͷؒʹΓཱͭදతͳؔࣜΛநग़͍ͯ͠Δͷ ͱͯ͠ଊ͑Δ͜ͱ͕Ͱ͖Δɻ
ͯ͞ɺҐ૬ۂ໘ͷʹΖ͏ɻ࣮ɺຊߘͰৄࡉͳઆ໌ɺূ໌͠ͳ͍͕ɺ §2 Ͱ ߟͨ͠ʢίϯύΫτ͔ͭ࿈݁ͳʣ
छg≥1Ґ૬ۂ໘S ͷ߹ɺઌఔઆ໌ͨ͠܈ίϗϞϩδʔͷཧద༻Մೳ
Ͱ͋Δɻͭ·ΓɺSͷίϗϞϩδʔՃ܈࣍ݩɺS ͷجຊ܈π1(S)ͷΈΛ༻͍ͯଊ͑Δ
͜ͱ͕Ͱ͖Δͱ͍͏͜ͱͰ͋Δɻ
§2.2 ͰɺҐ૬ۂ໘S্ͷදతͳྠମͦΕʹΑͬͯఆ·Δπ1(S) ͷੜݩʹ͍ͭ
ͯղઆ͕ͨ͠ɺͦͷΑ͏ͳੜݩͨͪɺ࣮
H1(π1(S)) →∼ H1(S)
ͷݩΛఆΊ͍ͯΔͷͰ͋ΔɻͦͷΑ͏ʹߟ͑Δͱɺ §2.2 ͰऔΓ্͛ͨπ1(S)ͷੜݩ
ͨͪɹ
α1, β1, α2, β2, . . . αg, βg
ͪΐ͏Ͳͦͷ··H1(S)ͷੜݩͷΛఆΊ͍ͯΔ͜ͱΛ؆୯ʹࣔ͢͜ͱ͕Ͱ͖Δɻ·
ͨɺ §2.2Ͱߟ͑ͨੜݩͷؒͷؔࣜ
α1·β1·α−11 ·β1−1·α2·β2·α−12 ·β2−1·. . .·αg·βg·α−1g ·βg−1 = 1
ͪΐ͏Ͳͦͷ··ɺ
H2(π1(S)) →∼ H2(S) ∼= Z ͷੜݩΛఆΊ͍ͯΔͷͰ͋Δɻ
ҰൠʹɺʢҐ૬ʣۭؒ܈ͷίϗϞϩδʔͷཧͰɺʮΧοϓੵʯ
: Hn(−) × Hm(−) → Hn+m(−)
ͱݺΕΔʮֻ͚ࢉͷΑ͏ͳʯૢ࡞͕ఆٛ͞ΕΔ͕ɺྫ͑ɺۭؒͷίϗϞϩδʔͷ߹ɺ
͜ͷΧοϓੵେࡶʹ͍͏ͱɺʮn࣍ݩͷ݀ʯͱʮm࣍ݩͷ݀ʯͷڞ௨෦ʹൃੜ͢Δز Կతͳঢ়گΛצఆ͢Δ͜ͱʹΑͬͯܭࢉ͞ΕΔͷͱݟΔ͜ͱ͕Ͱ͖Δɻ
ઌఔͷҐ૬ۂ໘Sͷ߹ɺα1 ͱβ1ͱ͍͏දతͳྠମʢਤ̒ΛࢀরʣɺҰճͷΈަ
ΘΓɺ͔ͦ͠ͷަΘΓํʮ͖Ε͍ͳेࣈܗʯʢʹֶ༻ޠͰ͍͏ͱʮԣஅతͳަࠩʯɺ
·ͨʮॏෳ1ͷަࠩʯʣʹΑΔͷͰ͋Δɻͭ·Γɺผͷݴ͍ํΛ͢Δͱɺྠମα1ͱ β1 བྷ·Γ߹͍ͬͯΔ͕ɺͦͷབྷ·Γ߹͍ํɺ࠷୯७ͳछྨͷབྷ·Γ߹͍ํͰ͋Δɻ
͜ͷΑ͏ͳঢ়گΛɺΧοϓੵΛ༻͍ͯදݱ͢Δͱɺ࣍ͷΑ͏ͳ͜ͱ͕ؼ݁Ͱ͖ΔɿΧοϓੵ
ɹ
α1 β1 ∈ H2(S) ∼= H2(π1(S))
ͪΐ͏Ͳ্ड़ͷؔࣜʹΑͬͯఆ·ΔH2(−)ͷੜݩͱҰக͢ΔͷͰ͋Δɻͭ·ΓɺΧο ϓੵα1
β1 ͪΐ͏Ͳ্ड़ͷؔࣜͷதͷʮα1ͱβ1͕ؔ͢Δ෦ʯΛநग़͍ͯ͠Δ ͱݟΔ͜ͱ͕Ͱ͖Δɻ
ਤ̒ɿʮॏෳ1ʯͰབྷ·Γ߹͏ɺҐ૬ۂ໘্ͷදతͳྠମ
§3.3. ମͷίϗϞϩδʔ࣍ݩ
܈ίϗϞϩδʔɺલઅʢ§3.2ʣͰऔΓ্͛ͨΑ͏ͳҐ૬ۂ໘ͷجຊ܈͚ͩͰͳ͘ɺ§2.3 Ͱղઆͨ͠Α͏ͳ෭༗ݶجຊ܈ §1.3Ͱհͨ͠Α͏ͳମͷઈରΨϩΞ܈ʹରͯ͠ఆ
ٛ͢Δ͜ͱՄೳͰ͋Δɻ·ͨɺͦͷΑ͏ʹ͢Δ͜ͱʹΑͬͯ෭༗ݶجຊ܈ମͷઈର ΨϩΞ܈ʹ͍ͭͯɺ܈ίϗϞϩδʔʹΑΔʮ࣍ݩʯͷఆ͕ٛՄೳʹͳΔɻҐ૬ۂ໘ͷ
߹ɺͦͷʮ࣍ݩʯ௨ৗͷجຊ܈ͷ܈ίϗϞϩδʔͰఆٛ͠Α͏͕ɺ෭༗ݶجຊ܈ͷ܈ί ϗϞϩδʔͰఆٛ͠Α͏͕ɺશ͘ಉ͡ʹͳΔɻͭ·Γɺྫ͑ʢίϯύΫτ͔ͭ࿈݁ͳʣ छg≥1Ґ૬ۂ໘ͷ߹ɺ࣍ݩ2ʹͳΔɻ
Ұํɺ͍͠ఆཧʢৄ͘͠[NSW], Proposition 8.3.17 ΛࢀরʣͰ͋Δ͕ɺi=√
−1 ͷಛघੑͷؔͰૉ2ʹ͓͍ͯൃੜ͢Δ͋Δܰඍͳʮٕज़తোʯΛআ͚ɺ࣮ɺ
ମF ʹਵ͢Δɹ
ઈରΨϩΞ܈GF ͷίϗϞϩδʔ࣍ݩ2ʹͳΔ
ͷͰ͋Δɻͭ·Γɺগͳ࣍͘ݩͱ͍͏ʮૈࡶͳෆมྔʯΛ௨ͯ͠ݟΔݶΓʹ͓͍ͯɺ
ମͷઈରΨϩΞ܈ͱछg ≥1ͷҐ૬ۂ໘ྨࣅ͍ͯ͠Δͱ͍͏͜ͱͰ͋Δɻ
Ұํɺछg≥1ͷҐ૬ۂ໘ͷʢ௨ৗͷʣجຊ܈ͱҧͬͯɺ§2.2Ͱհͨ͠Α͏ͳʮ͖
Ε͍ͳྠମʯʹΑΔ؆୯͔ͭ໌ࣔతͳੜݩͷؔࣜɺମͷઈରΨϩΞ܈ͷ߹
ʹɺ೦ͳ͕ΒΒΕ͍ͯͳ͍ɻ͔͠͠ɺछg≥ 1ͷҐ૬ۂ໘ͷجຊ܈ͱͷఆੑతͳ
ྨࣅੑʢʹ §3.1 ͷޙʹग़͖ͯͨʮ1࣍ݩͷ݀ʯʮ2࣍ݩͷ݀ʯʹؔ͢ΔٞΛࢀরʂʣ Λࣔ͢දతͳͷͱͯ͠ΫϯϚʔ֦େ(Kummer extension)ͷΨϩΞ܈ͱ͍͏؆୯͔ͭ
ॳతͳ۩ମྫ͕͋ΔͷͰ͜Εʹ͍ͭͯৄ͘͠આ໌͍ͨ͠ɻ
·ͣɺମF ͱૉpΛݻఆ͢Δɻ·ͨɺf ∈ F ɺF ʹpࠜΛ࣋ͨͳ͍ݩͱ͢
Δɻ͜ͷͱ͖ɺ
K = F(p
f) ⊆ Q
ͷΑ͏ͳ֦େମF ͷΫϯϚʔ֦େͱݺͿɻҰํɺ1ͷݪ࢝pࠜ
ω def= e2πi/p ∈ Q ⊆ C ΛF ʹఴՃ͢Δ͜ͱʹΑͬͯಘΒΕΔ֦େମ
L = F(ω) ⊆ Q
F ͷԁ֦େ(cyclotomic extension)ͱݺͿɻԁ֦େඞͣΨϩΞʹͳΔ͕ɺҰൠʹ
ΫϯϚʔ֦େΨϩΞ֦େʹͳΒͳ͍͜ͱ͋Δɻ͔͠͠ɺԁ֦େʹ্͕͔ͬͯΒఆ
ٛ͞ΕͨΫϯϚʔ֦େɺͭ·Γ
M = F(ω,p
f) ⊆ Q
ͷΑ͏ͳ֦େମඞͣF ͷΨϩΞ֦େʹͳΔɻྫ͑ɺ§1.3ͰऔΓ্֦͛ͨେ‘K/Q’ʢʹ ຊઅͷه߸Ͱɺp = 3, f = 2, F = Q ͷ߹ʹ૬ʣਖ਼ʹ͜ͷΑ͏ͳܗͷΨϩΞ֦େ
ͷಛผͳ߹ʹ֘͢Δɻ
Ұൠͷpͱf ∈F ͷʹΖ͏ɻ͜ͷ߹ɺGal(L/F)ҰͭͷݩͰੜ͞Εɺ͔ͭͦ
ͷݩͷҐʢʹͦͷݩΛn͢Ε୯ҐݩʹͳΔ࠷খͷn≥1ʣ͕p−1ΛׂΔʹͳ Δ͜ͱ؆୯ʹࣔͤΔɻྫ͑ɺF Λݻఆ͠ɺૉpΛಈ͔͢ͱɺ༗ݶݸͷྫ֎తͳpΛ আ͚ɺ͜ͷҐʢʹ[L:F]ʣඞͣͪΐ͏Ͳp−1ʹͳΔɻҐ͕ͲͷΑ͏ʹͳΖ͏ͱɺ
͜ͷΑ͏ͳGal(L/F)ͷੜݩɺʮK ʹ੍ݶͨ͠ͱ͖ɺ߃ࣸ૾idʹͳΔʯͱ͍͏݅
Λ՝͢ͱɺGal(M/F)ͷҐ[L:F]ͷݩ
σ ∈ Gal(M/F)
ʹҰҙʹ্࣋ͪ͛ΒΕΔ͜ͱ؆୯ʹࣔ͢͜ͱ͕Ͱ͖ΔɻҰํɺf ʹ՝ͨ݅͠ΑΓɺ Gal(M/L)ඞͣҐpͷݩ
τ ∈ Gal(M/L) (⊆ Gal(M/F))
Ͱੜ͞ΕΔ܈ʹͳΔ͜ͱͪʹै͏ɻ؆୯ͷͨΊɺ[L : F] = p −1 Ͱɺ n ∈ {1, . . . , p−1}͕ɺpΛ๏ͱֻ͚ͨ͠ࢉʹΑͬͯఆ·Δ܈
(Z/pZ)× def= (Z/pZ) \ {0}
ͷੜݩʹͳ͍ͬͯΔͱԾఆ͠Α͏ɻ͢ΔͱɺʢσΛnʹରԠ͢ΔΑ͏ʹ࠾ΕʣΨϩΞ܈
Gal(M/F)ͪΐ͏Ͳੜݩ
σ, τ ͱؔࣜ
σp−1 = τp = id, σ·τ ·σ−1 = τn
Ͱఆٛ͞ΕΔҐʢʹೱʣp(p−1)ͷ༗ݶ܈ʹͳΔɻ܈Gal(M/F)ɺ͋Δ͍GF ͷ܈
ίϗϞϩδʔʹؔ࿈͚ͯཧ͢Δͱɺ
ੜݩσͱτ ਖ਼ʹͦͷH1(−)ʹؔ͢Δݩ Ͱ͋Γɺ·ͨ
σͱτ ͷབྷ·Γ߹͍ํΛهड़͍ͯ͠Δʢ࠷ޙͷʣؔࣜɺ ɹ ͪΐ͏ͲH2(−)ͷੜݩʹରԠ͍ͯ͠Δ ͷͰ͋Δɻ
ਤ̓ɿԁ֦େͱΫϯϚʔ֦େ͕৫Γ͢ʮతͳॏෳ1ͷབྷ·Γ߹͍ʯ
ω j
ω k
ω i ω
f λ
ͭ·Γɺछg≥1Ґ૬ۂ໘ͷཧʢલઅ §3.2 ͷޙΛࢀরʣͱͷྨࣅͰ͍͏ͱɺ ੜݩσ, τ ͪΐ͏Ͳੜݩαi, βiʹରԠ͍ͯͯ͠ɺ
·ͨ
σͱτ ͷབྷ·Γ߹͍ํΛهड़ͨؔࣜ͠
ͪΐ͏Ͳαi, βi ͨͪͷབྷ·Γ߹͍ํΛهड़ͨؔࣜ͠ʹରԠ͍ͯ͠Δ
ͱ͍͏;͏ʹղऍ͢Δ͜ͱ͕Ͱ͖Δʢʹ༷ʑͳ1ͷϕΩࠜf ͷʮλϕΩͨͪʯΛࣔ͠
ͨਤ̓Λࢀরʣɻ
§4. ମͱҐ૬ۂ໘ͷʮབྷ·Γ߹͍ͷݱʯɿମ্ͷۂઢ
§4.1. ମ্ͷۂతۂઢ
͜Ε·ͰຊߘͰମͱҐ૬ۂ໘ͷͦΕͧΕͷجૅతͳཧʹ͍ͭͯղઆ͕ͨ͠ɺந
తͳྨࣅੑͱ͔͘ɺͦΕͧΕͷཧʹ͍ͭͯݸผͷͷͱͯ͠ͷҐஔ͚ͰऔΓ্
͛ͨɻҰํɺزԿͷத৺తͳݚڀରͷҰͭͰ͋Δମ্Ͱఆٛ͞Εͨۂઢʢʹ
ུͯ͠ʮମ্ͷۂઢʯʣʹ͍ͭͯߟ͢ΔͱɺମͷཧͱҐ૬ۂ໘ͷཧͷ྆ํ͕
ۃΊͯݫີ͔ͭ໌ࣔతͳܗͰਂؔ͘ΘΓ߹͍ͬͯͯɺͦͷؔΘΓ߹͍ͷ༷ࢠΛݚڀ͠ղ໌
͢Δ͜ͱʹΑͬͯ྆ऀͷߏʹؔ͢Δ༷ʑͳ৽͍͠ݟ͕ಘΒΕΔɻ
·ͣʮʢࣹӨʣଟ༷ମʯʹ͍ͭͯઆ໌͢Δඞཁ͕͋Δɻn ≥ 1ʹରͯ͠ɺෳ
ૉମC্ͷʮn࣍ݩࣹӨۭؒʯ(n-dimensional projective space)࣍ͷΑ͏ͳಉྨͷ
ू߹ͱͯ͠ఆٛ͢Δ͜ͱ͕Ͱ͖Δɿ
PnC def= { (x0, x1, . . . , xn) ∈ Cn+1 }/∼ ʢͨͩ͠ɺಉؔ‘∼’
(x0, x1, . . . , xn) ∼ (y0, y1, . . . , yn)
⇐⇒ 0= ∃λ∈C s.t. (x0, x1, . . . , xn) = (λ·y0, λ·y1, . . . , λ·yn)
ͱ͍͏;͏ʹఆٛ͞ΕΔʣɻʮࣹӨଟ༷ମʯ(projective algebraic variety) X ͱɺز
͔ͭͷɺʢn+ 1ݸͷෆఆݩʹؔ͢Δʣෳૉͷ੪࣍ଟ߲ࣜ(homogeneous polynomial) ɹ
{ fi(T0, T1, . . . , Tn) }i∈I
ʢͨͩ͠ɺI ҙͷू߹Ͱɺʮ੪࣍ଟ߲ࣜʯͱɺͯ͢ͷ߲ͷ͕࣍Ұக͢Δଟ߲ࣜͷ
͜ͱʣͷڞ௨ͷྵશମ͔ΒͳΔPnCͷʢดʣ෦ू߹Ͱ͋ΔɻຊߘͰɺʮಛҟʯΛ࣋
ͨͳ͍ɺ͍ΘΏΔʮΒ͔ʯͳʢࣹӨʣଟ༷ମ͔͠ొ͠ͳ͍ɻʢࣹӨʣଟ༷ମʹ ରͯ͠ɺͦͷʮ࣍ݩʯͱ͍͏֓೦Λఆٛ͢Δ͜ͱՄೳ͕ͩɺ͜ΕΛਖ਼͘͠ఆٛ͢ΔͨΊ ʹߴڃͳʮՄʯ͕ඞཁʹͳΔͨΊɺຊߘͰৄ͘͠આ໌͠ͳ͍ɻͨͩɺຊߘ Ͱجຊతʹ࣍ݩ1ͷΒ͔͔ͭ࿈݁ͳࣹӨଟ༷ମɺͭ·Γʮۂઢʯ(algebraic
curve)͔͠ొ͠ͳ͍ɻ࣍ʹɺCͷ෦ମF ⊆C͕༩͑ΒΕͨͱ͠Α͏ɻઌఔͷఆٛʹग़
͖ͯͨ੪࣍ଟ߲ࣜͨͪ{fi}i∈I ͕ͯ͢F ʹΛ࣋ͭଟ߲ࣜʹ࠾ΕΔͱ͖ɺʢࣹӨʣ
ଟ༷ମX ʹରͯ͠ʮF ্Ͱఆٛ͞ΕΔʯ ͱ͍͏ݴ͍ํΛ͢Δɻɹ
ྫ͑ɺn= 2ͷͱ͖ɺP2CΛʮࣹӨฏ໘ʯͱݺͿ͜ͱ͋Δ͕ɺࣹӨฏ໘ͷ߹ɺҰͭ
ͷʢదͳ݅Λຬͨ͢ʣ੪࣍ଟ߲ࣜ
f(T0, T1, T2)
ʹΑͬͯۂઢ͕ఆ·Δɻ͜ͷ߹ɺʮΒ͔ʯͱ͍͏ੑ࣭ɺͦͷଟ߲ࣜͷภඍͨͪ
∂f
∂T0, ∂f
∂T1, ∂f
∂T2
ͱf ͷڞ௨ͷྵ(0,0,0)͔͠ͳ͍ͱ͍͏݅ʹରԠ͍ͯ͠Δɻྫ͑ɺ༗໊ͳʮϑΣ ϧϚͷํఔࣜʯ
f(T0, T1, T2) = T0d+T1d−T2d
ʢͨͩ͠ɺd≥1ʣʢ؆୯ʹ֬ೝͰ͖ΔΑ͏ʹʣ͜ͷ݅Λຬ͍ͨͯ͠ΔͨΊɺʢQ
্Ͱఆٛ͞ΕΔʂʣΒ͔ͳۂઢΛఆΊ͍ͯΔɻ ҰൠͷnͷʹΖ͏ɻۂઢ
X ⊆ PnC
͕༩͑ΒΕͨͱ͠Α͏ɻ͜ͷۂઢͷʮߏʯʢʹPnC ͷதͷຒΊࠐΈɺX ͷఆ
ٛʹ༻͍ΒΕͨ੪࣍ଟ߲ࣜͨͪʣΛΕͯɺԼ෦ͷҐ૬ۭؒͷΈߟ͑ΔΑ͏ʹ͢Δͱɺ
ͦͷҐ૬ۭ͕ؒʢίϯύΫτ͔ͭ࿈݁ͳʣछg≥0Ґ૬ۂ໘ʹͳΔ͜ͱ
ൺֱత؆୯ʹࣔ͢͜ͱ͕Ͱ͖Δɻछg͕2Ҏ্ʹͳΔ߹ಛʹॏཁͰɺͦͷ߹ʹ
ۂઢXΛۂతۂઢ(hyperbolic algebraic curve)ͱݺͿɻྫ͑ɺࣹӨฏ໘ͷ
߹ɺXͷఆٛํఔࣜf ͷ͕࣍dͩͱ͢Δͱɺ࣍ͷΑ͏ͳެࣜ
g = 1
2(d−1)(d−2)
ॳతزԿֶΛ༻͍Δ͜ͱʹΑͬͯ؆୯ʹূ໌͢Δ͜ͱ͕Ͱ͖Δɻͭ·ΓɺX ͷ
ۂੑd≥4ͱ͍͏݅ͱಉʹͳΔɻ
Ґ૬ۂ໘ͷ߹ɺ§2.3Ͱղઆͨ͠ීวඃ෴ͷΑ͏ͳʢҰൠʹແݶ࣍ͷʣඃ෴ɺ༷ʑ ͳඃ෴͕ଘࡏ͢ΔΘ͚͕ͩɺ
ଟ߲ࣜͰఆٛ͞ΕΔʮతͳੈքʯʹཹ·Ζ͏ͱ͢Δͱɺ
༗ݶ࣍ͷඃ෴͔͠ѻ͏͜ͱ͕Ͱ͖ͳ͍ɻ
ͭ·ΓɺۂઢXʹΑͬͯఆ·ΔҐ૬ۂ໘ͷʢ࿈݁ͳʣ༗ݶ࣍ඃ෴ɺݩͷXͱಉ༷ɺ
ۂઢͱͯࣗ͠વʹఆٛ͞ΕΔ͕ɺແݶ࣍ඃ෴ʹ͍ͭͯಉ༷ͳੑཱ࣭͠ͳ͍ɻ
ۂઢXͷ༗ݶ࣍ͷඃ෴͕తʹఆٛ͞ΕΔͱ͍͏͜ͱɺ§2.3ͰऔΓ্͛ͨʮ෭
༗ݶجຊ܈ʯ ‘π1(−)’ X ʹΑͬͯఆ·ΔҐ૬ۂ໘ʹରͯ͠ఆٛͰ͖ɺ͔ͦ͠ΕΛɺ
͋Δۂઢͷʹग़ͯ͘Δ
ͦΕͧΕͷۂઢͷʢ༗ݶͳʂʣඃ෴ม܈ͨͪͷ͢
ܥͷٯۃݶͱͯ͠ѻ͏͜ͱ͕Ͱ͖Δ ͱ͍͏͜ͱͰ͋Δɻ͜ͷ෭༗ݶجຊ܈Λ
π1(X)
ͱද͢͜ͱʹ͢Δɻ
࣍ʹɺX͕ମ F ্Ͱఆٛ͞Ε͍ͯΔͱ͠Α͏ɻ͢Δͱɺઌఔͷʮۂઢͷʯʹ
ొ͢Δ֤ʑͷۂઢͨͪɺʢF ্Ͱఆٛ͞ΕΔͱݶΒͳ͍͕ʣF ͷదͳ༗ݶ֦࣍
େʢ⊆Qʣͷ্Ͱఆٛ͞ΕΔ͜ͱ؆୯ʹࣔͤΔɻैͬͯɺF ͷઈରΨϩΞ܈GF Λɺ͜
ΕΒͷۂઢͷఆٛํఔࣜͨͪʹ͋ΒΘΕΔͨͪʢʹQͷݩʂʣʹ࡞༻ͤ͞Δ͜ͱ ʹΑͬͯɺGF Λ্ड़ͷʮۂઢͷʯʹ࡞༻ͤ͞Δ͜ͱ͕Ͱ͖Δʢਤ̔Λࢀরʣɻ
ਤ̔ɿମͷઈରΨϩΞ܈Ґ૬ۂ໘ͷ෭༗ݶجຊ܈ʹࣗવʹ֎࡞༻͢Δ
෭༗ݶجຊ܈π1(X)ɺݫີʹ͍͏ͱ෦ࣗݾಉܕΛআ͍͔ͯ͠ఆٛ͞Εͳ͍ͷͳͷ Ͱɺ͜ͷΑ͏ͳGF ͷʮ֎࡞༻ʯ(outer action)ʹΑͬͯ
ρX : GF → Out(π1(X))
ͷΑ͏ͳܗͷࣗવͳ४ಉܕʹʮ֎෦දݱʯ(outer representation) ͕ఆ·Δɻ͜ͷGF ͷ
π1(X)ͷ֎࡞༻ɺ
. . .
G F
外作用
ମͷʢʹGFʣͱҐ૬ۂ໘ͷҐ૬زԿʢʹ෭༗ݶجຊ܈π1(X)ʣͱ͍͏ɺ Ұݟશ͘ҟ࣭ͳೋछྨͷֶతߏΛؔ࿈͚Δ
ॏཁͳݚڀରͰ͋Δɻɹ
§4.2. ෭༗ݶجຊ܈ͷઈରΨϩΞ܈ͷ࣮ͳ֎࡞༻
લઅʢ§4.1ʣͷ֎෦දݱρX ʹ͍༷ͭͯʑͳ͔֯Βଟछଟ༷ͳݚڀ͕ߦͳΘΕ͍ͯ
Δ͕ɺρX ʹ͍ͭͯΒΕ͍ͯΔ࠷جຊతͳࣄ࣮ͷҰͭ࣍ͷ݁Ռʢ[HM], Theorem C ΛࢀরʣͰ͋Δɻ
ఆཧɿମF ্Ͱఆٛ͞ΕΔۂతۂઢXʹਵ͢Δࣗવͳ֎෦දݱ ρX : GF → Out(π1(X))
୯ࣹʹͳΔɻ
ಉछͷʮ୯ࣹੑʯʹؔ͢Δఆཧɺʮ͕݀։͍͍ͯΔʯʹʮίϯύΫτͰͳ͍ʯۂత
ۂઢͷ߹ʹɺطʹ[Mtm]Ͱূ໌͞Ε͍ͯͯɺ[Mtm][HM]ɺҰ൪࠷ॳʹBelyi ࢯʹΑͬͯൃݟ͞ΕͨɺࣹӨઢP1 ͔ΒࡾΛൈ͍ͯಘΒΕΔۂతۂઢͷ߹ͷ୯ࣹ
ੑʹؼணͤ͞Δ͜ͱʹΑͬͯΑΓҰൠతͳۂతۂઢͷ߹ͷ୯ࣹੑΛূ໌͍ͯ͠Δɻ Ұํɺ্هͷ ఆཧ ͷΑ͏ʹίϯύΫτͳۂతۂઢͷ߹ʹ͜ͷछͷ୯ࣹੑΛࣔ͢͜
ͱͷҙٛɺ §3.2ٴͼ §3.3 Ͱղઆͨ͠Α͏ʹɺ
ίϯύΫτͳछgͷҐ૬ۂ໘ͱମͷઈରΨϩΞ܈ʹɺ ʮೋ࣍ݩతͳ܈తབྷ·Γ߹͍ʯͱ͍͏
ਂ͍ߏతྨࣅੑ͕͋ΓɺͦͷΑ͏ͳྨࣅੑΛ࣋ͭɺҰݟશ͘ҟ࣭ͳ
తͳରͱҐ૬زԿֶతͳରΛؔ࿈͚͍ͯΔ͜ͱʹ͋Δɻ
ͭ·Γɺ্هͷ ఆཧ ɺతͳํͷʮೋ࣍ݩతͳ܈తབྷ·Γ߹͍ʯ͕ɺͦͷࣗવͳ֎
࡞༻ʹΑͬͯҐ૬زԿֶతͳํͷʮೋ࣍ݩతͳ܈తབྷ·Γ߹͍ʯʹ࣮ʹදݱ͞Ε͍ͯ
Δ͜ͱΛݴ͍ͬͯΔͷͰ͋Δɻผͷݴ͍ํΛ͢Δͱɺ७ਮʹʮՄʯͷࢹʢʹͭ·
Γɺͬͱ۩ମతͳݴ༿Ͱ͍͏ͱɺॳతͳՃݮআͷൣᙝʣͰߟ͢Δͱɺମͱۂత
ۂઢ͍ͣΕ࣍ݩ1ͷରͰ͋Γɺ͔ͦ͠ͷతͳߏʢʹͭ·Γɺਖ਼ʹʮՃ ݮআʯͷߏʣશ͘ҟ࣭Ͱ͋Δ͕ɺΨϩΞ܈෭༗ݶجຊ܈ͷʮೋ࣍ݩతͳ܈తབྷ
·Γ߹͍ʯΛ௨ͯ྆͠ऀΛߟ͢Δ͜ͱʹΑͬͯɺʢ§3.2 ٴͼ §3.3 Ͱղઆͨ͠Α͏ͳʣਂ
͍ߏతͳྨࣅੑ͕ු͔ͼ্͕Γɺ·্ͨهͷ ఆཧ ͷ୯ࣹੑʹΑͬͯͦͷ྆ऀͷܨ͕ΓΛ ۃΊͯ໌ࣔతͳܗͰఆࣜԽ͢Δ͜ͱ͕ՄೳʹͳΔɻ
ࢀߟจݙ
[Mtm] M. Matsumoto, Galois representations on profinite braid groups on curves, J. Reine Angew. Math. 474 (1996), pp. 169-219.
[NSW] J. Neukirch, A. Schmidt, K. Wingberg,Cohomology of number fields,Grundlehren der Mathematischen Wissenschaften323, Springer-Verlag (2000).
[HM] Y. Hoshi, S. Mochizuki, On the combinatorial anabelian geometry of nodally nonde- generate outer representations, Hiroshima Math. J. 41(2011), pp. 275-342.