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Most likely square for a one-or-two jumping frog

Question

A frog sits at 00 on the number line and jumps forward repeatedly, each jump being 11 or 22 units with probability 12\tfrac12 each. For k>0k>0, let pkp_k be the probability the frog ever lands exactly on kk. What is the maximum of pkp_k over all k>0k>0?

Short answer

3/43/4. Position kk is reached from k−1k-1 or k−2k-2, so pk=12pk−1+12pk−2p_k=\tfrac12p_{k-1}+\tfrac12p_{k-2} with p0=1p_0=1, p1=12p_1=\tfrac12. Solving gives pk=23+13(−12)kp_k=\tfrac23+\tfrac13\left(-\tfrac12\right)^k, which is largest at k=2k=2: 34\tfrac34.

Adapted from QuantQuestions.io; question and answer rewritten by SelfMadeQuant.com.

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