Compact balance spring of constant section

A one-piece balance spring comprising:

• only a single strip that includes a plurality of loops wound on itself, the single strip being wound between an inner coil loop of the single strip and an outer coil loop of the single strip, each coil loop of the single strip is immediately adjacent to a next coil loop of the single strip from the inner coil loop of the single strip to a penultimate coil loop of the single strip, wherein the single strip further includes
• a first area between an end of the inner coil loop of the single strip and a start of the second coil loop of the single strip, wherein a pitch decreases in the first area,
• a second area in an extension of the first area, wherein, in the second area, the pitch between each coil loop of the single strip continuously increases, and each coil loop of the single strip from the inner coil loop of the single strip to the penultimate coil loop of the single strip has a constant distance between each coil loop of the single strip when an angle of contraction of the balance spring has a value of 360 degrees, and
• a third area, in an extension of the second area and comprised between a start of the penultimate coil loop of the single strip and an end of the outer coil loop of the single strip, wherein the pitch between the penultimate coil loop of the single strip and the outer coil loop of the single strip continuously increases in the third area.