The OEIS is supported by the many generous donors to the OEIS Foundation. Hints (Greetings from The On-Line Encyclopedia of Integer Sequences!)
 A319506 Number of numbers of the form 2*p or 3*p between consecutive triangular numbers T(n - 1) < {2,3}*p <= T(n) with p prime. 1
 0, 0, 2, 2, 2, 1, 2, 2, 2, 2, 3, 2, 3, 2, 3, 4, 3, 3, 3, 4, 5, 2, 4, 3, 5, 5, 2, 6, 3, 5, 5, 5, 5, 6, 4, 3, 7, 5, 6, 6, 5, 6, 7, 4, 5, 6, 6, 7, 6, 7, 9, 6, 6, 7, 8, 5, 6, 7, 9, 7, 7, 8, 7, 11, 7, 8, 8, 7, 6, 11, 5, 12, 7, 7, 7, 11, 11, 7, 12, 10, 9, 10, 7, 9, 9, 8, 10, 12, 10, 7, 10, 9, 12, 9, 11, 10, 13, 14, 10, 7 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,3 COMMENTS 1) It is conjectured that for k >= 1 each left-sided half-open interval (T(2*k - 1), T(2*k + 1)] and (T(2*k), T(2*(k + 1))] contains at least one composite c_2 = 2*p_i and c_3 = 3*p_j each, p_i, p_j prime, i != j. 2) It is conjectured that for k >= 3 each left-sided half-open interval (T(k - 1), T(k)] contains at least one composite c_2 = 2*p_i or c_3 = 3*p_j, p_i, p_j prime, i != j. 3) It is conjectured that for k >= 2 each left-sided half-open interval (T(2*k - 1), T(2*k)] contains at least one composite c_3 = 3*p_j, p_j prime. 4) It is conjectured that for k >= 1 each left-sided half-open interval (T(2*k), T(2*k + 1)] contains at least one composite c_2 = 2*p_i, p_i prime. LINKS Harvey P. Dale, Table of n, a(n) for n = 1..1000 EXAMPLE a(3) = 2 since (T(3 - 1),T(3)] = {4 = 2*2,5,6 = 2*3 = 3*2}, 2,3 prime. MATHEMATICA Table[Count[   Select[Range[(n - 1) n/2 + 1, n (n + 1)/2],    PrimeQ[#/2] || PrimeQ[#/3] &], _Integer], {n, 1, 100}] p23[{a_, b_}]:=Module[{r=Range[a+1, b]}, Count[Union[Join[r/2, r/3]], _?PrimeQ]]; p23/@Partition[Accumulate[Range[0, 100]], 2, 1] (* Harvey P. Dale, May 02 2020 *) PROG (PARI) isok1(n, k) = ((n%k) == 0) && isprime(n/k); isok2(n) = isok1(n, 2) || isok1(n, 3); t(n) = n*(n+1)/2; a(n) = sum(i=t(n-1)+1, t(n), isok2(i)); \\ Michel Marcus, Oct 12 2018 CROSSREFS Cf. A000040 (primes), A000217 (triangular numbers). Sequence in context: A278570 A046799 A348172 * A037809 A280534 A129451 Adjacent sequences:  A319503 A319504 A319505 * A319507 A319508 A319509 KEYWORD nonn AUTHOR Ralf Steiner, Sep 21 2018 STATUS approved

Lookup | Welcome | Wiki | Register | Music | Plot 2 | Demos | Index | Browse | More | WebCam
Contribute new seq. or comment | Format | Style Sheet | Transforms | Superseeker | Recents
The OEIS Community | Maintained by The OEIS Foundation Inc.

Last modified September 24 16:41 EDT 2022. Contains 356943 sequences. (Running on oeis4.)