is seen from (1), § 6, to be an invariant. If we multiply this by the invariants
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we obtain the invariants
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(1)
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(2)
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(3)
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(4)
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and the constitutive relations are obtained by making the first of these equal to μ times the third, and the second equal to ε times the fourth, where ε and μ are invariants. These, however, are not the only constitutive relations which remain invariant,[1] for we may obtain the two integral invariants
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(5)
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(6)
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A set of constitutive relations given by two linear relations between
- ↑ This was pointed out to me by Mr. Hassé,