= Adams-operation obstruction to retracting a truncated complex projective space
{c}
Let $\mathbb{CP}^{k+n}_k=\mathbb{CP}^{k+n}/\mathbb{CP}^{k-1}$. If the bottom-cell inclusion $S^{2k}=\mathbb{CP}^k_k\to\mathbb{CP}^{k+2}_k$ admits a retraction, then $24$ divides $k$. Writing $x=[\overline\gamma]-1$, a retracted Bott generator has the form $x^k+ax^{k+1}+bx^{k+2}$. Comparing it with its image under $\psi^2(x)=2x+x^2$ gives $a=-k/2$ and $b=k(3k+5)/24$; integrality forces both $3$ and $8$ to divide $k$.
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