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 A088821 a(n) is the sum of smallest prime factors of numbers from 1 to n. 6
 0, 2, 5, 7, 12, 14, 21, 23, 26, 28, 39, 41, 54, 56, 59, 61, 78, 80, 99, 101, 104, 106, 129, 131, 136, 138, 141, 143, 172, 174, 205, 207, 210, 212, 217, 219, 256, 258, 261, 263, 304, 306, 349, 351, 354, 356, 403, 405, 412, 414, 417, 419, 472, 474, 479, 481, 484 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,2 REFERENCES M. Kalecki, On certain sums extended over primes or prime factors, Prace Mat, Vol. 8 (1963), pp. 121-127. J. Sandor, D. S. Mitrinovic, B. Crstici, Handbook of Number Theory I, Volume 1, Springer, 2005, Chapter IV, p. 121. LINKS Michael De Vlieger, Table of n, a(n) for n = 1..10000 Project Euler, Problem 521 - Smallest prime factor FORMULA a(n) ~ n^2/(2 log n) [Kalecki]. - Thomas Ordowski, Nov 29 2018 a(n) = Sum_{prime p} n(p)*p, where n(p) is the number of integers in [1,n] with smallest prime factor spf(.) = A020639(.) = p, decreasing from n(2) = floor(n/2) to n(p) = 1 for p >= sqrt(n), possibly earlier, and n(p) = 0 for p > n. One has n(p) ~ D(p)*n where D(p) = (Product_{primes q < p} 1-1/q)/p = A038110/A038111 is the density of numbers having p as smallest prime factor. - M. F. Hasler, Dec 05 2018 MATHEMATICA Prepend[Accumulate[Rest[Table[FactorInteger[i][[1, 1]], {i, 60}]]], 0] (* Harvey P. Dale, Jan 09 2011 *) PROG (PARI) a(n) = sum(k=2, n, factor(k)[1, 1]); \\ Michel Marcus, May 15 2017 (GAP) P:=List(List([2..60], n->Factors(n)), i->i);; a:=Concatenation(, List([1..Length(P)], i->Sum([1..i], k->P[k]))); # Muniru A Asiru, Nov 29 2018 CROSSREFS Cf. A020639, A046669, A088822, A088823, A088824, A088825. Sequence in context: A007445 A159699 A063217 * A057897 A294146 A022758 Adjacent sequences:  A088818 A088819 A088820 * A088822 A088823 A088824 KEYWORD nonn AUTHOR Labos Elemer, Oct 22 2003 STATUS approved

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Last modified May 11 16:38 EDT 2021. Contains 343803 sequences. (Running on oeis4.)