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 A154297 Primes of the form (1+2+3+...+m)/21 = A000217(m)/21, for some m. 3
 5, 11, 41, 43 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 COMMENTS This asks for primes p which are a triangular number divided by 21, or, 2*3*7*p=k*(k+1) for some k. Matching factors shows that the sequence is complete [R. J. Mathar, Aug 15 2010] Original definition: Primes of the form : 1/x+2/x+3/x+4/x+5/x+6/x+7/x+..., x=21. The corresponding m-values are m=14, 21, 41, 42 (cf. A154296).  It is clear that for m>42, A000217(m)/21 = m(m+1)/42 cannot be a prime. - M. F. Hasler, Dec 31 2012 LINKS MATHEMATICA lst={}; s=0; Do[s+=n/21; If[Floor[s]==s, If[PrimeQ[s], AppendTo[lst, s]]], {n, 0, 9!}]; lst #/21&/@Select[Accumulate[Range[100]], PrimeQ[#/21]&] (* Harvey P. Dale, Dec 17 2012 *) PROG (PARI) select(x->denominator(x)==1 & isprime(x), vector(42, m, m^2+m)/42)  \\ - M. F. Hasler, Dec 31 2012 CROSSREFS Cf. A057570, A154293, A154296, A154298, A154299, A154300, A154301, A154302, A154303, A154304. Sequence in context: A276663 A187984 A063626 * A089441 A046121 A023271 Adjacent sequences:  A154294 A154295 A154296 * A154298 A154299 A154300 KEYWORD nonn,fini,full,easy AUTHOR Vladimir Joseph Stephan Orlovsky, Jan 06 2009 EXTENSIONS Added keywords fini,full - R. J. Mathar, Aug 15 2010 Edited by M. F. Hasler, Dec 31 2012 STATUS approved

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Last modified January 22 10:23 EST 2021. Contains 340362 sequences. (Running on oeis4.)