login
The OEIS is supported by the many generous donors to the OEIS Foundation.

 

Logo
Hints
(Greetings from The On-Line Encyclopedia of Integer Sequences!)
A140864 Smallest odd number with same number of divisors as 3*a(n-1). 2
1, 3, 9, 15, 45, 105, 315, 945, 2835, 3465, 10395, 31185, 45045, 135135, 405405, 675675, 2027025, 3828825, 11486475, 34459425, 72747675, 218243025, 654729075, 1527701175, 4583103525, 11712375675, 35137127025, 105411381075 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,2

LINKS

Dimitri Papadopoulos, Table of n, a(n) for n = 1..56

EXAMPLE

9*3=27 has 4 divisors, but smallest odd number with 4 divisors is 15.

PROG

(PARI) a(nn) = {ia = 1; print1(ia, ", "); for (n = 1, nn - 1, nd = numdiv(3*ia); forstep(i = 1, 3*ia, 2, if (numdiv(i) == nd, ia = i; break; ); ); print1(ia, ", "); ); } \\ Michel Marcus, Jun 14 2013

(PARI) {/*prints b-file for A140864 - add more for loops for more terms*/ print("#A140864"); print(1" "1); print(2" "3); n = 3; for(p=3, 56, tau = numdiv(3*n); exp3n=factor(n)[1, 2]; delta = bigomega(exp3n+2) - bigomega(exp3n+1); delta = max(delta+1, 2); var = exp3n+delta; num = 10^1000; for( n1=1, var, for (n2=0, n1, for( n3=0, n2, for( n4=0, n3, for( n5=0, n4, for( n6=0, n5, for( n7=0, n6, for( n8=0, n7, for( n9=0, n8, for( n10=0, n9, for( n11=0, n10, for( n12=0, n11, for( n13=0, n12, for( n14=0, n13, for( n15=0, n14, if( (n1+1) * (n2+1) * (n3+1) * (n4+1) * (n5+1) * (n6+1) * (n7+1) * (n8+1) * (n9+1) * (n10+1) * (n11+1) * (n12+1) * (n13+1) * (n14+1) * (n15+1) == tau, numtemp = prime(2)^n1 * prime(3)^n2 * prime(4)^n3 * prime(5)^n4 * prime(6)^n5 * prime(7)^n6 * prime(8)^n7 * prime(9)^n8 * prime(10)^n9 * prime(11)^n10 * prime(12)^n11 * prime(13)^n12 * prime(14)^n13 * prime(15)^n14 * prime(16)^n15; if(numtemp < num, num = numtemp); )); ); ); ); ); ); ); ) ; ); ); ); ) ; ); ); ); print(p" "num); n=num; )} \\ Dimitri Papadopoulos, May 08 2019

CROSSREFS

Cf. A053624, A019505. d(a(n)) = A036451(n) for first 18 terms.

Sequence in context: A053624 A348198 A119239 * A171929 A188597 A330815

Adjacent sequences: A140861 A140862 A140863 * A140865 A140866 A140867

KEYWORD

nonn

AUTHOR

J. Lowell, Jul 20 2008

EXTENSIONS

a(10) through a(28) from Klaus Brockhaus, Jul 23 2008

a(29) through a(56) from Dimitri Papadopoulos, May 08 2019

STATUS

approved

Lookup | Welcome | Wiki | Register | Music | Plot 2 | Demos | Index | Browse | More | WebCam
Contribute new seq. or comment | Format | Style Sheet | Transforms | Superseeker | Recents
The OEIS Community | Maintained by The OEIS Foundation Inc.

License Agreements, Terms of Use, Privacy Policy. .

Last modified February 4 10:27 EST 2023. Contains 360053 sequences. (Running on oeis4.)