\begin{document}$ J/\psi $\end{document} and \begin{document}$ \psi(3770) $\end{document} has to be strong enough if X(6900) is of the molecule type. We argue that since \begin{document}$ \psi(3770) $\end{document} decays predominantly into a \begin{document}$ D\bar D $\end{document} pair, the interactions between \begin{document}$ J/\psi $\end{document} and \begin{document}$ \psi(3770) $\end{document} may be significantly enhanced owing to the three point \begin{document}$ D\bar D $\end{document} loop diagram. The enhancement originates from the anomalous threshold located at \begin{document}$ t=-1.288 $\end{document} GeV\begin{document}$ ^2 $\end{document}, whose effect propagates into the s-channel partial wave amplitude in the vicinity of \begin{document}$ \sqrt{s}\simeq 6.94 $\end{document} GeV. This effect may be helpful in the formation of the \begin{document}$ X(6900) $\end{document} peak."> A discussion on the anomalous threshold enhancement of <inline-formula><tex-math id="M1">\begin{document}$ {\boldsymbol J/\boldsymbol\psi }$\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="//www.macurncorp.com/hepnp/article/app/id/4042e61f-7e4e-41ae-9a78-9d9e6ae0b957/CPC-2023-0551_M1.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="//www.macurncorp.com/hepnp/article/app/id/4042e61f-7e4e-41ae-9a78-9d9e6ae0b957/CPC-2023-0551_M1.png"/></alternatives></inline-formula> –<inline-formula><tex-math id="M2">\begin{document}${ \boldsymbol\psi \bf (3770) }$\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="//www.macurncorp.com/hepnp/article/app/id/4042e61f-7e4e-41ae-9a78-9d9e6ae0b957/CPC-2023-0551_M2.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="//www.macurncorp.com/hepnp/article/app/id/4042e61f-7e4e-41ae-9a78-9d9e6ae0b957/CPC-2023-0551_M2.png"/></alternatives></inline-formula> couplings and <inline-formula><tex-math id="M3">\begin{document}${\boldsymbol X \bf (6900) }$\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="//www.macurncorp.com/hepnp/article/app/id/4042e61f-7e4e-41ae-9a78-9d9e6ae0b957/CPC-2023-0551_M3.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="//www.macurncorp.com/hepnp/article/app/id/4042e61f-7e4e-41ae-9a78-9d9e6ae0b957/CPC-2023-0551_M3.png"/></alternatives></inline-formula> peak -
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