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 A119665 Sign in the term (2q +/- 1) for triangular numbers of the form q * (2q +/- 1) where both factors are primes (or prime powers). 0
 -1, 1, -1, 1, -1, 1, -1, 1, -1, 1, -1, 1, -1, 1, 1, -1, 1, -1, -1, 1, -1, -1, 1, -1, -1, -1, 1, -1, 1, -1, -1, -1, 1, 1, 1, -1, 1, -1, 1, 1, 1, 1, -1, -1, -1, 1, 1, -1, 1, -1, -1, -1, 1, 1, 1, 1, -1, 1, -1, 1, -1, -1, -1, -1, 1, -1, -1, 1, -1, 1, -1, 1, 1, -1, 1, -1, -1, 1, -1, -1, -1, -1, 1, 1, 1, -1, 1, -1, 1, -1, 1, 1, 1, 1, -1, -1, -1, -1, 1, -1, 1, -1, -1 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1 COMMENTS As always, 1 is considered to be a prime power. LINKS EXAMPLE a(1) = -1 since  1 = 1*1 and 1 = 2*1 - 1, a(2) = +1 since  3 = 1*3 and 3 = 2*1 + 1, a(3) = -1 since  6 = 2*3 and 3 = 2*2 - 1, a(4) = +1 since 10 = 2*5 and 5 = 2*2 + 1, a(5) = -1 since 15 = 3*5 and 5 = 2*3 - 1, a(6) = +1 since 21 = 3*7 and 7 = 2*3 + 1, a(7) = -1 since 28 = 4*7 and 7 = 2*4 - 1, a(8) = +1 since 36 = 4*9 and 9 = 2*4 + 1, a(9) = -1 since 45 = 5*9 and 9 = 2*5 - 1, a(10) = 1 since 55 = 5*11 and 11 = 2*5 + 1; 66 and 78 are the first triangular numbers not equal to a product of prime powers q*(2q+-1); a(11) = -1 since 91 = 7*13 and 13 = 2*7 - 1; 105 and 120 aren't of the required form, either; a(12) = +1 since 136 = 8*17 and 17 = 2*8 + 1, a(13) = -1 since 153 = 9*17 and 17 = 2*9 - 1, a(14) = +1 since 171 = 9*19 and 19 = 2*9 + 1; now 190, 210 and 231 aren't of the required form, which yields the first a(n) = a(n-1): a(15) = +1 since 253 = 11*23 and 23 = 2*11 + 1. - M. F. Hasler, Apr 21 2015 PROG (PARI) for(q=1, 999, (isprimepower(q)||q==1)&&forstep(j=-1, 1, 2, (isprimepower(q*2+j)||q*2+j==1)&&print1(j", "))) \\ M. F. Hasler, Apr 21 2015 CROSSREFS Cf. A068443, A000217. Sequence in context: A143431 A064179 A065357 * A121241 A122188 A158388 Adjacent sequences:  A119662 A119663 A119664 * A119666 A119667 A119668 KEYWORD sign AUTHOR Greg Huber, Jul 28 2006 EXTENSIONS Missing and wrong terms and offset corrected; more terms added by M. F. Hasler, Apr 21 2015 STATUS approved

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Last modified June 26 13:04 EDT 2022. Contains 354883 sequences. (Running on oeis4.)