A213653 Least semiprime whose digital sum is n, or 0 if no such integer exists.

0, 10, 10001, 21, 4, 14, 6, 25, 26, 9, 46, 38, 39, 49, 77, 69, 169, 278, 0, 289, 299, 489, 589, 689, 699, 799, 899, 0, 2899, 3899, 4989, 5899, 5999, 6999, 7999, 9899, 0, 19999, 29999, 48999, 58999, 68999, 69999, 88999, 99899, 0, 299899, 398999, 589989

Offset

0,2

Comments

a(9k) = 0 for all k>1.

I conjecture that all terms > 278, except for 10001, end in the digit "9". What is the next term a(n) > 69 violating monotony, i.e., such that a(n) < a(n-1)? M. F. Hasler, Jun 17 2012

a(88) = 7999999999 < a(87) = 8899899999. - Alois P. Heinz, Jun 17 2012

a(76) = 499999999 < a(75) = 597999999. - Donovan Johnson, Jun 18 2012

Links

Alois P. Heinz, Table of n, a(n) for n = 0..400 (first 101 terms from Donovan Johnson)

Mathematica

semiPrimeQ[n_] := PrimeOmega[n] == 2; t = Table[0, {100}]; k = 1; While[k < 10^7, If[ semiPrimeQ@ k, s = Plus @@ IntegerDigits@ k; If[s < 101 && t[[s]] == 0, t[[s]] = k; Print[{s, k}]]]; k++]

Prog

(PARI) A213653(n)={ n%9 || n==9 || return; forstep( a=A051885(n), 9e9, 9, bigomega(a)==2 || next; A007953(a)==n & return(a))} \\ - M. F. Hasler, Jun 17 2012

Crossrefs

Sequence in context: A163019 A013746 A076782 * A283889 A283890 A139109

Adjacent sequences: A213650 A213651 A213652 * A213654 A213655 A213656

Keyword

easy,base,nonn

Author

Robert G. Wilson v, Jun 17 2012

Extensions

a(0)-a(62) double-checked with given PARI code by M. F. Hasler, Jun 17 2012

Status

approved