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 A212848 Least prime factor of n-th central trinomial coefficient (A002426). 1
 1, 1, 3, 7, 19, 3, 3, 3, 3, 43, 7, 3, 113, 73, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 7, 17, 3, 719, 7, 3, 3, 3, 3, 967, 9539, 3, 17, 47, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 19 (list; graph; refs; listen; history; text; internal format)
 OFFSET 0,3 COMMENTS A002426(n) is prime for n = 2, 3, 4, no more through 10^5. A002426 is semiprime iff A102445(n) = 2 (as is the case for n = 5, 6, 7, 9, 10, 12, 13). LINKS Robert Israel, Table of n, a(n) for n = 0..729 FORMULA a(n) = A020639(A002426(n)). EXAMPLE a(9) = 43 because A002426(9) = 3139 = 43 * 73. MAPLE A002426:= gfun:-rectoproc({(n+2)*a(n+2)-(2*n+3)*a(n+1)-3*(n+1)*a(n) = 0, a(0)=1, a(1)=1}, a(n), remember): lpf:= proc(n) local F; F:= map(proc(t) if t[1]::integer then t[1] else NULL fi end proc, ifactors(n, easy)[2]); if nops(F) > 0 then min(F) else min(numtheory:-factorset(n)) fi end proc: lpf(1):= 1: map(lpf @ A002426, [\$0..100]); # Robert Israel, Jun 20 2017 MATHEMATICA a = b = 1; t = Join[{a, b}, Table[c = ((2 n - 1) b + 3 (n - 1) a)/n; a = b; b = c; c, {n, 2, 100}]]; Table[FactorInteger[n][[1, 1]], {n, t}] (* T. D. Noe, May 30 2012 *) PROG (PARI) a(n) = my(x=polcoeff((1 + x + x^2)^n, n)); if (x==1, 1, vecmin(factor(x)[, 1])); \\ Michel Marcus, Jun 20 2017 CROSSREFS Cf. A000040, A002426, A020639, A102445, A212791. Sequence in context: A173449 A270519 A361087 * A217371 A088629 A075609 Adjacent sequences: A212845 A212846 A212847 * A212849 A212850 A212851 KEYWORD nonn,easy AUTHOR Jonathan Vos Post, May 28 2012 STATUS approved

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