1,2

a(n) is a subset of A002181(n) - Inverse of Euler totient function. Most a(n) are primes except for n=12,21,32,33,34,40,... a(12) = 85 = 5*17, a(21) = 185 = 5*37, a(32) = 289 = 17*17, a(33) = 295 = 5*59, a(34) = 305 = 5*61, a(40) = 335 = 5*67,... Each composite a(n) appears to be a product of two primes from previous a(n) or a square of a prime from previous a(n).

Composite a(n) are the products of powers of previous primes from the a(n). For example, there are a(n) equal to 17, 17^2, 5*17^2, 59^2, 59*61, 61^2, 61*67, 67^2, 67*73, 17^3, 5*17*59, 71*73, 5*17*61, 73^2, 71*79, 73*79, 5*17*73, 79^2, 61*167, 101^2, 37*277, 5*37*59, 79*139, 107^2, 5*17*139, 5*37*67, 5*37*71, 17^2*47, 61*223, 61*227, 5*17*163, 5*17*167, 71*227, 127^2, 17^2*59, 5*59^2, 17^2*61, 5*61^2, 137^2, 137*139, 139^2, 17^2*67, 5*17*229, 137*149, 5*61*67, 5*59*71, 17^2*73, 5*67^2, 5*61*79, 5*67*73, 5*17^3... - Alexander Adamchuk (alex(AT)kolmogorov.com), Jul 28 2006

a(n) = A051230[n]/10. a(n) = A051229[n]/5.

Do[s=1+Divisors[n]; s1=Flatten[Position[PrimeQ[s], True]]; s2=Part[s, s1]; If[Equal[s2, {2, 3, 11}], Print[n/10]], {n, 1, 50000}] - Alexander Adamchuk (alex(AT)kolmogorov.com), Jul 28 2006

Cf. A051230, A051229, A002445, A002181.

Adjacent sequences: A119453 A119454 A119455 this_sequence A119457 A119458 A119459

Sequence in context: A121325 A080167 A060245 this_sequence A053755 A107199 A048209

nonn

Alexander Adamchuk (alex(AT)kolmogorov.com), Jul 26 2006

More terms from Alexander Adamchuk (alex(AT)kolmogorov.com), Jul 28 2006