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A167790
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a(n) is the index k of k-th prime p(k) in the smallest sum s(k)=2+3+...+p(k)=t*p(n) of first k primes where t is a true divisor and first occurrence of factor p(n) (n=1,2,3,...)
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2
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3, 10, 3, 5, 8, 49, 13, 23, 23, 7, 39, 29, 15, 10, 39, 25, 30, 151, 38, 19, 139, 27, 174, 21, 287, 422, 240, 24, 94, 22, 16, 173, 861, 231, 143, 140, 213, 902, 18, 134, 143, 310, 70, 58, 295, 550, 237, 210, 229, 57, 221, 271, 194, 540, 145, 718, 116, 184, 90, 71, 168
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OFFSET
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1,1
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COMMENTS
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It is conjectured that the sequence is infinite
If t is not restricted to nontrivial divisors, the sequence becomes A111287. - R. J. Mathar, Nov 17 2009
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REFERENCES
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Richard E. Crandall, Carl Pomerance: Prime Numbers, Springer 2005
Leonard E. Dickson: History of the Theory of numbers, vol. I, Dover Publications 2005)
Paulo Ribenboim, The New Book of Prime Number Records, Springer 1996
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LINKS
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Table of n, a(n) for n=1..61.
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FORMULA
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a(n) = min[2+3+...+p(k)/t], where the minimum is taken with respect to k, the denominator t > 1 is an integer divisor of numerator s(k)=2+3+...+p(k).
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EXAMPLE
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(1) s(5)=2+3+5+7+11=28=2^2*7=4*p(4) gives a(4)=5 as first occurrence of prime factor p(4)=7;
(2) s(8)=2+3+5+7+11+13+17+19=77=7*11=7*p(5) gives a(5)=8 as first occurrence of prime factor p(5)=11;
(3) s(422)=2+3+5+...+2917=570145= 5 * 101 * 1129=5645*p(26) gives a(26)=422 and demonstrates the numerical difficulties.
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CROSSREFS
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Cf. A000504 (sum of first n primes).
Cf. A167764.
Sequence in context: A124692 A091043 A321118 * A010708 A072988 A170855
Adjacent sequences: A167787 A167788 A167789 * A167791 A167792 A167793
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KEYWORD
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nonn
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AUTHOR
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Eva-Maria Zschorn (e-m.zschorn(AT)zaschendorf.km3.de), Nov 12 2009, Nov 13 2009
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EXTENSIONS
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Extended by R. J. Mathar, Nov 17 2009
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STATUS
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approved
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