| Step | Hyp | Ref | Expression | 
						
							| 1 |  | isumshft.1 |  | 
						
							| 2 |  | isumshft.2 |  | 
						
							| 3 |  | isumshft.3 |  | 
						
							| 4 |  | isumshft.4 |  | 
						
							| 5 |  | isumshft.5 |  | 
						
							| 6 |  | isumshft.6 |  | 
						
							| 7 | 5 4 | zaddcld |  | 
						
							| 8 | 2 | eleq2i |  | 
						
							| 9 | 4 | zcnd |  | 
						
							| 10 |  | eluzelcn |  | 
						
							| 11 | 10 2 | eleq2s |  | 
						
							| 12 | 1 | fvexi |  | 
						
							| 13 | 12 | mptex |  | 
						
							| 14 | 13 | shftval |  | 
						
							| 15 | 9 11 14 | syl2an |  | 
						
							| 16 |  | simpr |  | 
						
							| 17 |  | eqid |  | 
						
							| 18 | 17 | fvmpt2i |  | 
						
							| 19 | 16 18 | syl |  | 
						
							| 20 |  | eluzelcn |  | 
						
							| 21 | 20 1 | eleq2s |  | 
						
							| 22 |  | addcom |  | 
						
							| 23 | 9 21 22 | syl2an |  | 
						
							| 24 |  | id |  | 
						
							| 25 | 24 1 | eleqtrdi |  | 
						
							| 26 |  | eluzadd |  | 
						
							| 27 | 25 4 26 | syl2anr |  | 
						
							| 28 | 23 27 | eqeltrd |  | 
						
							| 29 | 28 2 | eleqtrrdi |  | 
						
							| 30 |  | eqid |  | 
						
							| 31 | 3 30 | fvmpti |  | 
						
							| 32 | 29 31 | syl |  | 
						
							| 33 | 19 32 | eqtr4d |  | 
						
							| 34 | 33 | ralrimiva |  | 
						
							| 35 |  | nffvmpt1 |  | 
						
							| 36 | 35 | nfeq1 |  | 
						
							| 37 |  | fveq2 |  | 
						
							| 38 |  | oveq2 |  | 
						
							| 39 | 38 | fveq2d |  | 
						
							| 40 | 37 39 | eqeq12d |  | 
						
							| 41 | 36 40 | rspc |  | 
						
							| 42 | 34 41 | mpan9 |  | 
						
							| 43 | 42 | ralrimiva |  | 
						
							| 44 | 5 | adantr |  | 
						
							| 45 | 4 | adantr |  | 
						
							| 46 |  | simpr |  | 
						
							| 47 | 46 2 | eleqtrdi |  | 
						
							| 48 |  | eluzsub |  | 
						
							| 49 | 44 45 47 48 | syl3anc |  | 
						
							| 50 | 49 1 | eleqtrrdi |  | 
						
							| 51 |  | fveq2 |  | 
						
							| 52 |  | oveq2 |  | 
						
							| 53 | 52 | fveq2d |  | 
						
							| 54 | 51 53 | eqeq12d |  | 
						
							| 55 | 54 | rspccva |  | 
						
							| 56 | 43 50 55 | syl2an2r |  | 
						
							| 57 |  | pncan3 |  | 
						
							| 58 | 9 11 57 | syl2an |  | 
						
							| 59 | 58 | fveq2d |  | 
						
							| 60 | 15 56 59 | 3eqtrrd |  | 
						
							| 61 | 8 60 | sylan2br |  | 
						
							| 62 | 7 61 | seqfeq |  | 
						
							| 63 | 62 | breq1d |  | 
						
							| 64 | 13 | isershft |  | 
						
							| 65 | 5 4 64 | syl2anc |  | 
						
							| 66 | 63 65 | bitr4d |  | 
						
							| 67 | 66 | iotabidv |  | 
						
							| 68 |  | df-fv |  | 
						
							| 69 |  | df-fv |  | 
						
							| 70 | 67 68 69 | 3eqtr4g |  | 
						
							| 71 |  | eqidd |  | 
						
							| 72 | 6 | fmpttd |  | 
						
							| 73 | 72 | ffvelcdmda |  | 
						
							| 74 | 2 7 71 73 | isum |  | 
						
							| 75 |  | eqidd |  | 
						
							| 76 | 29 | ralrimiva |  | 
						
							| 77 | 38 | eleq1d |  | 
						
							| 78 | 77 | rspccva |  | 
						
							| 79 | 76 78 | sylan |  | 
						
							| 80 |  | ffvelcdm |  | 
						
							| 81 | 72 79 80 | syl2an2r |  | 
						
							| 82 | 42 81 | eqeltrd |  | 
						
							| 83 | 1 5 75 82 | isum |  | 
						
							| 84 | 70 74 83 | 3eqtr4d |  | 
						
							| 85 |  | sumfc |  | 
						
							| 86 |  | sumfc |  | 
						
							| 87 | 84 85 86 | 3eqtr3g |  |