“…As the period function TðhÞ is given by Abelian integral (1.2), it is natural that the initial problem can be reduced to estimate the number of zeros of Abelian integral dTðhÞ=dh: A simple but important fact is that dTðhÞ=dh can be expressed as a linear combination of two basic integrals which satisfy a Picard-Fuchs equation. Some results concerned with the study of the number of zeros of Abelian integrals can be found in [CsH,F1,Gl3,I1,I2,NY,P,RZh,Zh,ZLL,ZS,ZZ2] and references therein.…”