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I have the following expressions:

R[1, 1] R[2, 1] R[3, 1] R[4, 1] 
sum[FF[3, ak[1] - ak[2], 1], EE[2, ak[2] - ak[3], 0], EE[2, ak[3] - ak[4], 0]]

By what procedure can I convert such expressions into expression:

sum[R[1, 1], FF[3, ak[2] - ak[2], 1], R[2, 1] , EE[2, ak[2] - ak[3], 0], R[3, 1] , EE[2, ak[3] - ak[4], 0] , R[4, 1]]
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  • $\begingroup$ Perhaps you should look at Riffle. $\endgroup$
    – m_goldberg
    Feb 28, 2019 at 14:29

1 Answer 1

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expr = R[1, 1] R[2, 1] R[3, 1] R[4, 1] sum[FF[3, ak[1] - ak[2], 1], 
    EE[2, ak[2] - ak[3], 0], EE[2, ak[3] - ak[4], 0]];

expr /. a_ sum[b__] :> sum @@ Riffle[List @@ a, {b}]

sum[R[1, 1], FF[3, ak[1] - ak[2], 1], R[2, 1], EE[2, ak[2] - ak[3], 0], R[3, 1], EE[2, ak[3] - ak[4], 0], R[4, 1]]

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