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 A128301 Indices of squares (of primes) in the semiprimes. 15
 1, 3, 9, 17, 40, 56, 90, 114, 164, 253, 289, 404, 484, 533, 634, 783, 973, 1031, 1233, 1373, 1452, 1683, 1842, 2112, 2483, 2676, 2779, 2995, 3108, 3320, 4124, 4384, 4775, 4926, 5593, 5741, 6172, 6644, 6962, 7448, 7955, 8108, 8978, 9147, 9512, 9697, 10842 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,2 COMMENTS A001358(a(n)) = A001248(n) = A000040(n)^2. Numbers n with property that tau(semiprime(n)) is not semiprime. - Juri-Stepan Gerasimov, Oct 15 2010 LINKS Zak Seidov, Table of n, a(n) for n = 1..10000 EXAMPLE a(4) = 17 as 49 = 7^2 = prime(4)^2, the fourth square in the semiprimes, is the seventeenth semiprime. MATHEMATICA With[{sp=Select[Range[50000], PrimeOmega[#]==2&]}, Flatten[Table[ Position[ sp, Prime[ n]^2], {n, Floor[Sqrt[Length[sp]]]}]]] (* Harvey P. Dale, Nov 17 2014 *) PROG (Perl) -MMath::Pari=factorint, PARI -wle 'my \$c = 0; my \$s = PARI 1; while (1) { ++\$s; my(\$sp, \$si) = @{factorint(\$s)}; next if @\$sp > 2; next if \$si->[0] + (@\$si > 1 ? \$si->[1] : 0) != 2; ++\$c; print "\$s => \$c" if @\$sp == 1}' # Hugo van der Sanden, Sep 25 2007 (PARI) a(n)=my(s=0, i=0); n=prime(n)^2; forprime(p=2, sqrt(n), s+=primepi(n\p); i++); s - i * (i-1)/2 \\ Charles R Greathouse IV, Apr 21 2011 CROSSREFS Cf. A128302, A001358, A001248, A072000. Sequence in context: A293423 A011755 A262466 * A348382 A176148 A206701 Adjacent sequences:  A128298 A128299 A128300 * A128302 A128303 A128304 KEYWORD nonn AUTHOR Rick L. Shepherd, Feb 25 2007 STATUS approved

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Last modified January 20 10:33 EST 2022. Contains 350472 sequences. (Running on oeis4.)