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 A082650 Number of primes < n of form 1+k*spf(n), where spf(n) is the smallest prime factor of n (A020639). 0
 0, 0, 0, 1, 0, 2, 0, 3, 1, 3, 0, 4, 0, 5, 2, 5, 0, 6, 0, 7, 3, 7, 0, 8, 1, 8, 3, 8, 0, 9, 0, 10, 4, 10, 2, 10, 0, 11, 5, 11, 0, 12, 0, 13, 6, 13, 0, 14, 2, 14, 6, 14, 0, 15, 3, 15, 6, 15, 0, 16, 0, 17, 7, 17, 4, 17, 0, 18, 8, 18, 0, 19, 0, 20, 9, 20, 3, 20, 0, 21, 10, 21, 0, 22, 5, 22, 10, 22, 0 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,6 LINKS FORMULA a(2*n)=A000720(2*n)-1; a(n)=0 iff n=1 or n prime, a(A008578(n))=1. - Reinhard Zumkeller, Sep 11 2003 EXAMPLE n=20, spf(20)=2: 8 primes of form 1+k*2: 1+1*2=3, 1+2*2=5, 1+3*2=7, 1+5*2=11, 1+6*2=13, 1+8*2=17 and 1+9*2=19, therefore a(20)=8; n=21, spf(21)=3: 3 primes of form 1+k*3: 1+2*3=7, 1+4*3=13 and 1+6*3=19, therefore a(21)=3. CROSSREFS Cf. A035096. Sequence in context: A199470 A098006 A336916 * A054875 A324115 A029239 Adjacent sequences:  A082647 A082648 A082649 * A082651 A082652 A082653 KEYWORD nonn AUTHOR Reinhard Zumkeller, May 16 2003 STATUS approved

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Last modified September 18 04:07 EDT 2021. Contains 347506 sequences. (Running on oeis4.)