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Description: Final equation of https://crypto.stanford.edu/pbc/notes/numberfield/fermatn4.html . Given A ^ 4 + B ^ 4 = C ^ 2 , provide a smaller solution. This satisfies the infinite descent condition. (Contributed by SN, 24-Aug-2024)
| Ref | Expression | ||
|---|---|---|---|
| Hypotheses | flt4lem5a.m | |- M = ( ( ( sqrt ` ( C + ( B ^ 2 ) ) ) + ( sqrt ` ( C - ( B ^ 2 ) ) ) ) / 2 ) |
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| flt4lem5a.n | |- N = ( ( ( sqrt ` ( C + ( B ^ 2 ) ) ) - ( sqrt ` ( C - ( B ^ 2 ) ) ) ) / 2 ) |
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| flt4lem5a.r | |- R = ( ( ( sqrt ` ( M + N ) ) + ( sqrt ` ( M - N ) ) ) / 2 ) |
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| flt4lem5a.s | |- S = ( ( ( sqrt ` ( M + N ) ) - ( sqrt ` ( M - N ) ) ) / 2 ) |
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| flt4lem5a.a | |- ( ph -> A e. NN ) |
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| flt4lem5a.b | |- ( ph -> B e. NN ) |
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| flt4lem5a.c | |- ( ph -> C e. NN ) |
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| flt4lem5a.1 | |- ( ph -> -. 2 || A ) |
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| flt4lem5a.2 | |- ( ph -> ( A gcd C ) = 1 ) |
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| flt4lem5a.3 | |- ( ph -> ( ( A ^ 4 ) + ( B ^ 4 ) ) = ( C ^ 2 ) ) |
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| Assertion | flt4lem5f | |- ( ph -> ( ( M gcd ( B / 2 ) ) ^ 2 ) = ( ( ( R gcd ( B / 2 ) ) ^ 4 ) + ( ( S gcd ( B / 2 ) ) ^ 4 ) ) ) |
| Step | Hyp | Ref | Expression |
|---|---|---|---|
| 1 | flt4lem5a.m | |- M = ( ( ( sqrt ` ( C + ( B ^ 2 ) ) ) + ( sqrt ` ( C - ( B ^ 2 ) ) ) ) / 2 ) |
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| 2 | flt4lem5a.n | |- N = ( ( ( sqrt ` ( C + ( B ^ 2 ) ) ) - ( sqrt ` ( C - ( B ^ 2 ) ) ) ) / 2 ) |
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| 3 | flt4lem5a.r | |- R = ( ( ( sqrt ` ( M + N ) ) + ( sqrt ` ( M - N ) ) ) / 2 ) |
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| 4 | flt4lem5a.s | |- S = ( ( ( sqrt ` ( M + N ) ) - ( sqrt ` ( M - N ) ) ) / 2 ) |
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| 5 | flt4lem5a.a | |- ( ph -> A e. NN ) |
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| 6 | flt4lem5a.b | |- ( ph -> B e. NN ) |
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| 7 | flt4lem5a.c | |- ( ph -> C e. NN ) |
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| 8 | flt4lem5a.1 | |- ( ph -> -. 2 || A ) |
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| 9 | flt4lem5a.2 | |- ( ph -> ( A gcd C ) = 1 ) |
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| 10 | flt4lem5a.3 | |- ( ph -> ( ( A ^ 4 ) + ( B ^ 4 ) ) = ( C ^ 2 ) ) |
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| 11 | 1 2 3 4 5 6 7 8 9 10 | flt4lem5d | |- ( ph -> M = ( ( R ^ 2 ) + ( S ^ 2 ) ) ) |
| 12 | 1 2 3 4 5 6 7 8 9 10 | flt4lem5e | |- ( ph -> ( ( ( R gcd S ) = 1 /\ ( R gcd M ) = 1 /\ ( S gcd M ) = 1 ) /\ ( R e. NN /\ S e. NN /\ M e. NN ) /\ ( ( M x. ( R x. S ) ) = ( ( B / 2 ) ^ 2 ) /\ ( B / 2 ) e. NN ) ) ) |
| 13 | 12 | simp2d | |- ( ph -> ( R e. NN /\ S e. NN /\ M e. NN ) ) |
| 14 | 13 | simp3d | |- ( ph -> M e. NN ) |
| 15 | 13 | simp1d | |- ( ph -> R e. NN ) |
| 16 | 13 | simp2d | |- ( ph -> S e. NN ) |
| 17 | 15 16 | nnmulcld | |- ( ph -> ( R x. S ) e. NN ) |
| 18 | 12 | simp3d | |- ( ph -> ( ( M x. ( R x. S ) ) = ( ( B / 2 ) ^ 2 ) /\ ( B / 2 ) e. NN ) ) |
| 19 | 18 | simprd | |- ( ph -> ( B / 2 ) e. NN ) |
| 20 | 14 | nnzd | |- ( ph -> M e. ZZ ) |
| 21 | 15 | nnzd | |- ( ph -> R e. ZZ ) |
| 22 | 20 21 | gcdcomd | |- ( ph -> ( M gcd R ) = ( R gcd M ) ) |
| 23 | 12 | simp1d | |- ( ph -> ( ( R gcd S ) = 1 /\ ( R gcd M ) = 1 /\ ( S gcd M ) = 1 ) ) |
| 24 | 23 | simp2d | |- ( ph -> ( R gcd M ) = 1 ) |
| 25 | 22 24 | eqtrd | |- ( ph -> ( M gcd R ) = 1 ) |
| 26 | 16 | nnzd | |- ( ph -> S e. ZZ ) |
| 27 | 20 26 | gcdcomd | |- ( ph -> ( M gcd S ) = ( S gcd M ) ) |
| 28 | 23 | simp3d | |- ( ph -> ( S gcd M ) = 1 ) |
| 29 | 27 28 | eqtrd | |- ( ph -> ( M gcd S ) = 1 ) |
| 30 | rpmul | |- ( ( M e. ZZ /\ R e. ZZ /\ S e. ZZ ) -> ( ( ( M gcd R ) = 1 /\ ( M gcd S ) = 1 ) -> ( M gcd ( R x. S ) ) = 1 ) ) |
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| 31 | 20 21 26 30 | syl3anc | |- ( ph -> ( ( ( M gcd R ) = 1 /\ ( M gcd S ) = 1 ) -> ( M gcd ( R x. S ) ) = 1 ) ) |
| 32 | 25 29 31 | mp2and | |- ( ph -> ( M gcd ( R x. S ) ) = 1 ) |
| 33 | 18 | simpld | |- ( ph -> ( M x. ( R x. S ) ) = ( ( B / 2 ) ^ 2 ) ) |
| 34 | 14 17 19 32 33 | flt4lem4 | |- ( ph -> ( M = ( ( M gcd ( B / 2 ) ) ^ 2 ) /\ ( R x. S ) = ( ( ( R x. S ) gcd ( B / 2 ) ) ^ 2 ) ) ) |
| 35 | 34 | simpld | |- ( ph -> M = ( ( M gcd ( B / 2 ) ) ^ 2 ) ) |
| 36 | 14 16 | nnmulcld | |- ( ph -> ( M x. S ) e. NN ) |
| 37 | 36 | nnzd | |- ( ph -> ( M x. S ) e. ZZ ) |
| 38 | 37 21 | gcdcomd | |- ( ph -> ( ( M x. S ) gcd R ) = ( R gcd ( M x. S ) ) ) |
| 39 | 23 | simp1d | |- ( ph -> ( R gcd S ) = 1 ) |
| 40 | rpmul | |- ( ( R e. ZZ /\ M e. ZZ /\ S e. ZZ ) -> ( ( ( R gcd M ) = 1 /\ ( R gcd S ) = 1 ) -> ( R gcd ( M x. S ) ) = 1 ) ) |
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| 41 | 21 20 26 40 | syl3anc | |- ( ph -> ( ( ( R gcd M ) = 1 /\ ( R gcd S ) = 1 ) -> ( R gcd ( M x. S ) ) = 1 ) ) |
| 42 | 24 39 41 | mp2and | |- ( ph -> ( R gcd ( M x. S ) ) = 1 ) |
| 43 | 38 42 | eqtrd | |- ( ph -> ( ( M x. S ) gcd R ) = 1 ) |
| 44 | 14 | nncnd | |- ( ph -> M e. CC ) |
| 45 | 16 | nncnd | |- ( ph -> S e. CC ) |
| 46 | 15 | nncnd | |- ( ph -> R e. CC ) |
| 47 | 44 45 46 | mul32d | |- ( ph -> ( ( M x. S ) x. R ) = ( ( M x. R ) x. S ) ) |
| 48 | 44 46 45 | mulassd | |- ( ph -> ( ( M x. R ) x. S ) = ( M x. ( R x. S ) ) ) |
| 49 | 48 33 | eqtrd | |- ( ph -> ( ( M x. R ) x. S ) = ( ( B / 2 ) ^ 2 ) ) |
| 50 | 47 49 | eqtrd | |- ( ph -> ( ( M x. S ) x. R ) = ( ( B / 2 ) ^ 2 ) ) |
| 51 | 36 15 19 43 50 | flt4lem4 | |- ( ph -> ( ( M x. S ) = ( ( ( M x. S ) gcd ( B / 2 ) ) ^ 2 ) /\ R = ( ( R gcd ( B / 2 ) ) ^ 2 ) ) ) |
| 52 | 51 | simprd | |- ( ph -> R = ( ( R gcd ( B / 2 ) ) ^ 2 ) ) |
| 53 | 52 | oveq1d | |- ( ph -> ( R ^ 2 ) = ( ( ( R gcd ( B / 2 ) ) ^ 2 ) ^ 2 ) ) |
| 54 | gcdnncl | |- ( ( R e. NN /\ ( B / 2 ) e. NN ) -> ( R gcd ( B / 2 ) ) e. NN ) |
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| 55 | 15 19 54 | syl2anc | |- ( ph -> ( R gcd ( B / 2 ) ) e. NN ) |
| 56 | 55 | nncnd | |- ( ph -> ( R gcd ( B / 2 ) ) e. CC ) |
| 57 | 56 | flt4lem | |- ( ph -> ( ( R gcd ( B / 2 ) ) ^ 4 ) = ( ( ( R gcd ( B / 2 ) ) ^ 2 ) ^ 2 ) ) |
| 58 | 53 57 | eqtr4d | |- ( ph -> ( R ^ 2 ) = ( ( R gcd ( B / 2 ) ) ^ 4 ) ) |
| 59 | 14 15 | nnmulcld | |- ( ph -> ( M x. R ) e. NN ) |
| 60 | 59 | nnzd | |- ( ph -> ( M x. R ) e. ZZ ) |
| 61 | 60 26 | gcdcomd | |- ( ph -> ( ( M x. R ) gcd S ) = ( S gcd ( M x. R ) ) ) |
| 62 | 26 21 | gcdcomd | |- ( ph -> ( S gcd R ) = ( R gcd S ) ) |
| 63 | 62 39 | eqtrd | |- ( ph -> ( S gcd R ) = 1 ) |
| 64 | rpmul | |- ( ( S e. ZZ /\ M e. ZZ /\ R e. ZZ ) -> ( ( ( S gcd M ) = 1 /\ ( S gcd R ) = 1 ) -> ( S gcd ( M x. R ) ) = 1 ) ) |
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| 65 | 26 20 21 64 | syl3anc | |- ( ph -> ( ( ( S gcd M ) = 1 /\ ( S gcd R ) = 1 ) -> ( S gcd ( M x. R ) ) = 1 ) ) |
| 66 | 28 63 65 | mp2and | |- ( ph -> ( S gcd ( M x. R ) ) = 1 ) |
| 67 | 61 66 | eqtrd | |- ( ph -> ( ( M x. R ) gcd S ) = 1 ) |
| 68 | 59 16 19 67 49 | flt4lem4 | |- ( ph -> ( ( M x. R ) = ( ( ( M x. R ) gcd ( B / 2 ) ) ^ 2 ) /\ S = ( ( S gcd ( B / 2 ) ) ^ 2 ) ) ) |
| 69 | 68 | simprd | |- ( ph -> S = ( ( S gcd ( B / 2 ) ) ^ 2 ) ) |
| 70 | 69 | oveq1d | |- ( ph -> ( S ^ 2 ) = ( ( ( S gcd ( B / 2 ) ) ^ 2 ) ^ 2 ) ) |
| 71 | gcdnncl | |- ( ( S e. NN /\ ( B / 2 ) e. NN ) -> ( S gcd ( B / 2 ) ) e. NN ) |
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| 72 | 16 19 71 | syl2anc | |- ( ph -> ( S gcd ( B / 2 ) ) e. NN ) |
| 73 | 72 | nncnd | |- ( ph -> ( S gcd ( B / 2 ) ) e. CC ) |
| 74 | 73 | flt4lem | |- ( ph -> ( ( S gcd ( B / 2 ) ) ^ 4 ) = ( ( ( S gcd ( B / 2 ) ) ^ 2 ) ^ 2 ) ) |
| 75 | 70 74 | eqtr4d | |- ( ph -> ( S ^ 2 ) = ( ( S gcd ( B / 2 ) ) ^ 4 ) ) |
| 76 | 58 75 | oveq12d | |- ( ph -> ( ( R ^ 2 ) + ( S ^ 2 ) ) = ( ( ( R gcd ( B / 2 ) ) ^ 4 ) + ( ( S gcd ( B / 2 ) ) ^ 4 ) ) ) |
| 77 | 11 35 76 | 3eqtr3d | |- ( ph -> ( ( M gcd ( B / 2 ) ) ^ 2 ) = ( ( ( R gcd ( B / 2 ) ) ^ 4 ) + ( ( S gcd ( B / 2 ) ) ^ 4 ) ) ) |