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A326151
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Numbers whose product of prime indices is twice their sum of prime indices.
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11
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49, 63, 65, 81, 150, 154, 190, 198, 364, 468, 580, 840, 952, 1080, 1224, 1480, 2128, 2288, 2736, 3440, 5152, 5280, 6624, 8480, 9408, 10816, 12096, 12992, 15552, 16704, 19520, 24960, 26752, 27776, 35712, 44800, 45440, 56576, 57600, 66304, 85248, 101120, 118272
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OFFSET
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1,1
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COMMENTS
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The only squarefree terms are 65, 154, and 190. See A326157 for a proof.
A prime index of n is a number m such that prime(m) divides n. The multiset of prime indices of n is row n of A112798.
The Heinz number of an integer partition (y_1,...,y_k) is prime(y_1)*...*prime(y_k), so these are Heinz numbers of integer partitions whose product of parts is twice their sum of parts. The enumeration of these partitions by sum is given by A326152.
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LINKS
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EXAMPLE
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The sequence of terms together with their prime indices begins:
49: {4,4}
63: {2,2,4}
65: {3,6}
81: {2,2,2,2}
150: {1,2,3,3}
154: {1,4,5}
190: {1,3,8}
198: {1,2,2,5}
364: {1,1,4,6}
468: {1,1,2,2,6}
580: {1,1,3,10}
840: {1,1,1,2,3,4}
952: {1,1,1,4,7}
1080: {1,1,1,2,2,2,3}
1224: {1,1,1,2,2,7}
1480: {1,1,1,3,12}
2128: {1,1,1,1,4,8}
2288: {1,1,1,1,5,6}
2736: {1,1,1,1,2,2,8}
3440: {1,1,1,1,3,14}
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MATHEMATICA
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primeMS[n_]:=If[n==1, {}, Flatten[Cases[FactorInteger[n], {p_, k_}:>Table[PrimePi[p], {k}]]]];
Select[Range[1000], Times@@primeMS[#]==2*Plus@@primeMS[#]&]
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PROG
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(PARI) is(k) = {my(f=factor(k)); for(i=1, #f~, f[i, 1]=primepi(f[i, 1])); factorback(f)==2*sum(i=1, #f~, f[i, 2]*f[i, 1]); } \\ Jinyuan Wang, Jun 27 2020
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CROSSREFS
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KEYWORD
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nonn
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AUTHOR
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STATUS
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approved
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