Prove that the number of partitions of $n$ into $3$ parts is equal to the number of partitions of $2n$ into $3$ parts, each of size less than $n$.

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Prove that the number of partitions of $n$ into $3$ parts is equal to the number of partitions of $2n$ into $3$ parts, each of size less than $n$.

I do not have any idea to prove this statement.Please help me.

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Outline: Let $P=(a,b,c)$ be a partition of $n$ into $3$ parts $1\le a\le b\le c$.

Let $\varphi(P)=(n-c,n-b,n-a)$. Show that $\varphi$ gives a one to one correspondence between the partitions of $n$ into $3$ parts and the partitions of $2n$ into $3$ parts with each part less than $n$.