权利要求：
1. A chair, comprising a seat portion in which a deformation mode of at least a portion of a seating surface is isolated such that a target curved surface including a double-curved surface of a prescribed type, relative to a prescribed position, is formed irrespective of a position of buttocks of a user when the user is seated.
2. The chair according to claim 1, wherein curvature of the double-curved surface depends on magnitude of a normal direction-component of an external force from the user, with respect to a reference surface of the seating surface.
3. The chair according to claim 2, wherein the double-curved surface is a bowl-type double-curved surface having a larger positive value of Gaussian curvature as the normal direction-component of the external force from the user, with respect to the reference surface of the seating surface, becomes larger.
4. The chair according to claim 3, wherein: the seat portion includes a plurality of unit structures continuously arranged along the reference surface of the seating surface, a first unit structure that is one of the plurality of unit structures includes: a first beam portion arranged substantially parallel to a main axis on the reference surface of the seating surface, the first beam portion passing through a first vertex among four or more vertices included in a polygon having four or more even numbers of sides; a second beam portion arranged substantially parallel to the main axis, the second beam portion passing through a second vertex among the four or more vertices, the second vertex being opposite to the first vertex; a connection portion connecting beam portions of a second unit structure and a third unit structure at a third vertex and a fourth vertex among the four or more vertices, the second unit structure and the third unit structure being adjacent to the first unit structure, the third vertex and the fourth vertex being opposite to each other; and column portions extending from the first beam portion and the second beam portion along the sides of the polygon, and Poisson's ratio when the first unit structure is pulled and deformed in a direction of the main axis is negative.
5. The chair according to claim 3, wherein: the seat portion includes a plurality of unit structures continuously arranged along the reference surface of the seating surface, at least one of the unit structures includes: a first beam portion including a first surface; a second beam portion including a second surface, the second surface being opposite to the first surface; first and second column portions extending from one end and the other end of the first surface toward the second surface, respectively, such that a distance between the first and second column portions decreases; third and fourth column portions extending from one end and the other end of the second surface toward the first surface, respectively, such that a distance between the third and fourth column portions decreases; and a connection portion connecting end portions of the first and second column portions and end portions of the third and fourth column portions, the end portions of the first and second column portions not being in contact with the first surface, the end portions of the third and fourth column portions not being in contact with the second surface, and the first beam portion and the second beam portion are thicker than the first column portion, the second column portion, the third column portion and the fourth column portion.
6. A chair, comprising: a seat portion; and a backrest in which a deformation mode of at least a portion of a back contact surface is isolated such that a first target curved surface including a double-curved surface of a first type, relative to a first position, is formed irrespective of a position of a back of a user when the user leans on the backrest.
7. The chair according to claim 6, wherein curvature of the double-curved surface depends on magnitude of a normal direction-component of an external force from the user, with respect to a reference surface of the back contact surface.
8. The chair according to claim 7, wherein the double-curved surface is a saddle-type double-curved surface having a smaller negative value of Gaussian curvature as the normal direction-component of the external force from the user, with respect to the reference surface of the back contact surface, becomes larger.
9. The chair according to claim 8, wherein: the backrest includes a plurality of unit structures continuously arranged along the reference surface of the back contact surface, a first unit structure that is one of the plurality of unit structures includes: a first beam portion arranged substantially parallel to a main axis on the reference surface of the back contact surface, the first beam portion passing through a first vertex among four or more vertices included in a polygon having four or more even numbers of sides; a second beam portion arranged substantially parallel to the main axis, the second beam portion passing through a second vertex among the four or more vertices, the second vertex being opposite to the first vertex; a connection portion connecting beam portions of a second unit structure and a third unit structure at a third vertex and a fourth vertex among the four or more vertices, the second unit structure and the third unit structure being adjacent to the first unit structure, the third vertex and the fourth vertex being opposite to each other; and column portions extending from the first beam portion and the second beam portion along the sides of the polygon, and Poisson's ratio when the first unit structure is pulled and deformed in a direction of the main axis is positive.
10. The chair according to claim 8, wherein: the backrest includes a plurality of unit structures continuously arranged along the reference surface of the back contact surface, at least one of the unit structures includes: a first beam portion including a first surface; a second beam portion including a second surface, the second surface being opposite to the first surface; first and second column portions extending from one end and the other end of the first surface toward the second surface, respectively, such that a distance between the first and second column portions increases; third and fourth column portions extending from one end and the other end of the second surface toward the first surface, respectively, such that a distance between the third and fourth column portions increases; and a connection portion connecting end portions of the first and second column portions and end portions of the third and fourth column portions, the end portions of the first and second column portions not being in contact with the first surface, the end portions of the third and fourth column portions not being in contact with the second surface, and the first beam portion and the second beam portion are thicker than the first column portion, the second column portion, the third column portion and the fourth column portion.
11. The chair according to any of claims 6 to 10, wherein a deformation mode of at least a portion of a seating surface of the seat portion is isolated such that a second target curved surface including a double-curved surface of a second type, relative to a second position, is formed irrespective of a position of buttocks of the user when the user is seated.
12. A seat portion, wherein a deformation mode of at least a portion of a seating surface is isolated such that a target curved surface including a double-curved surface of a prescribed type, relative to a prescribed position, is formed irrespective of a position of buttocks of a user when the user is seated.
13. A backrest, wherein a deformation mode of at least a portion of a back contact surface is isolated such that a target curved surface including a double-curved surface of a prescribed type, relative to a prescribed position, is formed irrespective of a position of a back of a user when the user leans on the backrest.