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A118699 Smaller of two consecutive semiprimes with the same digital root. 1
146, 226, 278, 346, 1018, 1177, 1273, 1546, 1594, 1865, 1945, 2138, 2545, 2762, 2798, 2869, 3118, 3317, 3817, 4022, 4045, 4069, 4126, 4198, 4213, 4249, 4322, 4417, 4502, 4511, 4918, 5114, 5389, 5438, 5473, 5515, 5645, 5677, 5855, 6169, 6209, 6218, 6362 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

LINKS

Nathaniel Johnston, Table of n, a(n) for n = 1..10000

FORMULA

A010888(a(n))=A010888(b(n)) where a(n)=A001358(k), b(n)=A001358(k+1). - R. J. Mathar, May 23 2006

EXAMPLE

146 and 155 are two consecutive semiprimes with the same digital root, namely 2.

MAPLE

with(numtheory): A001358 := proc(n) option remember: local k: if(n=1)then return 4:fi: for k from procname(n-1)+1 do if bigomega(k) = 2 then return k: fi: od: end: A118699ind := proc(n) option remember: local k: if(n=1)then return 50:fi: for k from procname(n-1)+1 do if(A001358(k) mod 9 = A001358(k+1) mod 9)then return k: fi: od: end: seq(A001358(A118699ind(n)), n=1..20); # Nathaniel Johnston, May 04 2011

MATHEMATICA

Select[Partition[Select[Range[6400], PrimeOmega[#]==2&], 2, 1], 1+Mod[#[[1]]-1, 9] == 1+Mod[#[[2]]-1, 9]&][[All, 1]] (* Harvey P. Dale, Oct 23 2019 *)

PROG

(PARI) isA001358(n)= { local(f, fct, sumpo); if(n <4, return(0) ); f=factor(n); fct=(matsize(f))[1]; sumpo= sum(i=1, fct, f[i, 2]); if ( sumpo != 2, return(0), return(1) ); } A010888(n)= { if( n %9 != 0, return(n % 9), return(9) ); } { oldr=0; oldn=0; for(n=5, 10000, if( isA001358(n)==1, dr=A010888(n); if(dr==oldr, print1(oldn, ", "); ); oldr=dr; oldn=n; ); ); } \\ R. J. Mathar, May 23 2006

CROSSREFS

Cf. A001358.

Sequence in context: A248406 A135666 A119379 * A238579 A238028 A179572

Adjacent sequences:  A118696 A118697 A118698 * A118700 A118701 A118702

KEYWORD

base,easy,nonn

AUTHOR

Luc Stevens (lms022(AT)yahoo.com), May 20 2006

EXTENSIONS

Corrected and extended by R. J. Mathar, May 23 2006

STATUS

approved

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Last modified May 30 13:58 EDT 2020. Contains 334724 sequences. (Running on oeis4.)