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A025129
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a(n) = p(1)p(n) + p(2)p(n-1) + ... + p(k)p(n-k+1), where k = [ n/2 ], p = A000040, the primes.
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16
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0, 6, 10, 29, 43, 94, 128, 231, 279, 484, 584, 903, 1051, 1552, 1796, 2489, 2823, 3784, 4172, 5515, 6091, 7758, 8404, 10575, 11395, 14076, 15174, 18339, 19667, 23414, 24906, 29437, 31089, 36500, 38614, 44731, 47071, 54198, 56914, 65051, 68371, 77402, 81052, 91341
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OFFSET
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1,2
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COMMENTS
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This is the sum of distinct squarefree semiprimes with prime indices summing to n + 1. A squarefree semiprime is a product of any two distinct prime numbers. A prime index of n is a number m such that the m-th prime number divides n. The multiset of prime indices of n is row n of A112798. - Gus Wiseman, Dec 05 2020
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LINKS
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FORMULA
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EXAMPLE
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The sequence of sums begins (n > 1):
6 = 6
10 = 10
29 = 14 + 15
43 = 22 + 21
94 = 26 + 33 + 35
128 = 34 + 39 + 55
231 = 38 + 51 + 65 + 77
279 = 46 + 57 + 85 + 91
(End)
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MATHEMATICA
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f[n_] := Block[{primeList = Prime@ Range@ n}, Total[ Take[ primeList, Floor[n/2]]*Reverse@ Take[ primeList, {Floor[(n + 3)/2], n}]]]; Array[f, 44] (* Robert G. Wilson v, Apr 07 2014 *)
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PROG
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(Haskell)
a025129 n = a025129_list !! (n-1)
a025129_list= f (tail a000040_list) [head a000040_list] 1 where
f (p:ps) qs k = sum (take (div k 2) $ zipWith (*) qs $ reverse qs) :
f ps (p : qs) (k + 1)
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CROSSREFS
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The nonsquarefree version is A024697 (shifted right).
Row sums of A338905 (shifted right).
A332765 is the greatest among these squarefree semiprimes.
A006881 lists squarefree semiprimes.
A014342 is the self-convolution of the primes.
A056239 is the sum of prime indices of n.
A339194 sums squarefree semiprimes grouped by greater prime factor.
Cf. A001221, A005117, A062198, A098350, A168472, A320656, A338900, A338901, A338904, A339114, A339116.
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KEYWORD
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nonn
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AUTHOR
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EXTENSIONS
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STATUS
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approved
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