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Aplicat¸ii liniare 65<br />
Propozit¸ia 3.16 (Suma a doi tensori). Dacă A i1i2...ip<br />
¸si Bi1i2...ip sunt coordonatele<br />
j1j2...jq j1j2...jq<br />
a doi tensori A ¸si B <strong>de</strong> tip (p, q) atunci<br />
T i1i2...ip<br />
= Ai1i2...ip + Bi1i2...ip<br />
j1j2...jq j1j2...jq j1j2...jq<br />
sunt coordonatele unui tensor <strong>de</strong> tip (p, q) notat cu A + B, adică<br />
Demonstrat¸ie (cazul p = q = 1). Avem<br />
(A + B) i1i2...ip<br />
= Ai1i2...ip + Bi1i2...ip<br />
j1j2...jq j1j2...jq j1j2...jq .<br />
T ′i<br />
j = A ′i<br />
j + B ′i<br />
j = β i k α m j A k m + β i k α m j B k m = β i k α m j (A k m + B k m) = β i k α m j T k m.<br />
Propozit¸ia 3.17 ( Înmult¸irea unui tensor cu un scalar). Dacă λ ∈ K ¸si Ai1i2...ip j1j2...jq<br />
sunt coordonatele unui tensor A <strong>de</strong> tip (p, q) atunci<br />
T i1i2...ip<br />
= λAi1i2...ip<br />
j1j2...jq j1j2...jq<br />
sunt coordonatele unui tensor <strong>de</strong> tip (p, q) notat cu λA, adică<br />
Demonstrat¸ie (cazul p = q = 1). Avem<br />
(λA) i1i2...ip<br />
= λAi1i2...ip<br />
j1j2...jq j1j2...jq .<br />
T ′i<br />
j = λA ′i<br />
j = λβ i k α m j A k m = β i k α m j T k m.<br />
Propozit¸ia 3.18 ( Produsul tensorial a doi tensori, într-un caz particular). Dacă<br />
A i jk sunt coordonatele unui tensor A <strong>de</strong> tip (1, 2) ¸si Bl m sunt coordonatele unui tensor<br />
B <strong>de</strong> tip (1, 1) atunci<br />
T il<br />
jkm = A i jk · B l m<br />
sunt coordonatele unui tensor <strong>de</strong> tip (2, 3) notat cu A ⊗ B, adică<br />
Demonstrat¸ie. Avem<br />
(A ⊗ B) il<br />
jkm = A i jk · B l m.<br />
T ′il<br />
jkm = A ′i<br />
jk · B ′l<br />
m = β i a α b j α c k A a bc β l r α s m B r s = β i a β l r α b j α c k α s m T ar<br />
bcs.