A060990 -id:A060990 - cp's OEIS frontend

A266110 If A082284(n) = 0, a(n) = 0, otherwise a(n) = 1 + a(A082284(n)), where A082284(n) = smallest number k such that k - d(k) = n, or 0 if no such number exists, and d(n) = the number of divisors of n (A000005).

4, 3, 4, 2, 1, 1, 3, 0, 0, 2, 2, 1, 3, 0, 1, 2, 1, 1, 2, 0, 0, 6, 1, 5, 0, 0, 2, 4, 0, 3, 4, 2, 1, 0, 3, 1, 0, 0, 2, 2, 0, 1, 2, 0, 1, 8, 1, 7, 1, 0, 0, 6, 0, 5, 3, 0, 0, 4, 2, 3, 7, 2, 1, 0, 0, 1, 0, 0, 0, 4, 1, 3, 6, 2, 0, 0, 8, 1, 4, 0, 1, 7, 3, 6, 5, 0, 2, 5, 0, 4, 12, 5, 1, 3, 11, 4, 4, 3, 0, 2, 0, 1, 10, 0, 3, 2, 9, 1, 0, 0, 1, 1, 0, 0, 8, 2, 0, 0, 7, 1, 2, 6

Offset: 0

Antti Karttunen, Dec 21 2015

nonn

Starting from n, search for a smallest number k such that k - d(k) = n, and if found such a number, replace n with k and repeat the procedure. When eventually such k is no longer found, then (new) n must be one of the terms of A045765. The number of times the procedure can be repeated before that happens is the value of a(n). Sequence A266116 gives the stopping value of n.

			Starting from n = 21, we get the following chain: 21 -> 23 -> 27 -> 29 -> 31 -> 35 -> 37, with A082284 iterated 6 times before the final term 37 (for which A060990(37) = A082284(37) = 0) is encountered. Thus a(21) = 6.

Antti Karttunen, Table of n, a(n) for n = 0..124340

One less than A266111.
Cf. A000005, A060990, A082284, A266116.
Cf. A045765 (positions of zeros).
Cf. tree A263267 (and its illustration).
Cf. also A264970.

A266116 The last nonzero term on each row of A265751.

Original entry on oeis.org

7, 7, 13, 7, 8, 7, 13, 7, 8, 13, 20, 13, 25, 13, 20, 19, 24, 19, 25, 19, 20, 37, 25, 37, 24, 25, 40, 37, 28, 37, 50, 37, 40, 33, 50, 37, 36, 37, 50, 43, 40, 43, 49, 43, 50, 67, 49, 67, 56, 49, 50, 67, 52, 67, 68, 55, 56, 67, 68, 67, 136, 67, 68, 63, 64, 67, 66, 67, 68, 79, 74, 79, 136, 79, 74, 75, 103, 79, 98, 79, 88, 103, 98, 103, 136, 85

Offset: 0

Antti Karttunen, Dec 21 2015

nonn

Starting from j = n, search for a smallest number k such that k - d(k) = j, and if found such a number, replace j with k and repeat the procedure. When eventually such k is no longer found, then the (last such) j must be one of the terms of A045765, and it is set as the value of a(n).

			Starting from n = 21, we get the following chain: 21 -> 23 -> 27 -> 29 -> 31 -> 35 -> 37, with A082284 iterated 6 times before the final term 37 (for which A060990(37) = A082284(37) = 0) is encountered. Thus a(21) = 37.

Antti Karttunen, Table of n, a(n) for n = 0..124340

Cf. A000005, A045765, A060990, A082284, A265751.
Cf. A266110 (gives the number of iterations of A082284 needed before a(n) is found).
Cf. also tree A263267 (and its illustration).

(definec (A266116 n) (cond ((A082284 n) => (lambda (lad) (if (zero? lad) n (A266116 lad))))))
;; Alternatively:
(define (A266116 n) (A265751bi n (A266110 n))) ;; Code for A265751bi given in A265751.

a(n) = A265751(n, A266110(n)).
If A060990(n) = 0, a(n) = n, otherwise a(n) = a(A082284(n)), where A082284(n) = smallest number k such that k - d(k) = n, or 0 if no such number exists, and d(n) = the number of divisors of n (A000005).
Other identities and observations. For all n >= 0:
a(n) >= n.
A060990(a(n)) = 0. [All terms are in A045765.]

A262900 a(n) = number of leaf-children n has in the tree generated by edge-relation A049820(child) = parent.

Original entry on oeis.org

0, 0, 0, 0, 1, 1, 0, 0, 0, 0, 0, 1, 0, 0, 1, 0, 1, 1, 0, 0, 0, 0, 2, 0, 0, 0, 0, 1, 0, 1, 0, 0, 1, 0, 0, 1, 0, 0, 0, 0, 0, 1, 0, 0, 1, 0, 2, 0, 1, 0, 0, 1, 0, 0, 0, 0, 0, 2, 1, 0, 0, 0, 1, 0, 0, 1, 0, 0, 0, 1, 1, 0, 0, 0, 0, 0, 0, 1, 0, 0, 1, 1, 0, 0, 0, 0, 0, 0, 0, 0, 0, 1, 1, 0, 0, 0, 1, 0, 0, 0, 0, 1, 1, 0, 0, 0, 0, 1, 0, 0, 1, 2, 0, 0, 1, 0, 0, 0, 1, 1, 1, 0

Offset: 0

Antti Karttunen, Oct 06 2015

nonn

a(n) = number of such terms k in A045765 for which k - d(k) = n [where d(k) is the number of divisors of k, A000005(k)].

			a(4) = 1, as there is only one such term k in A045765 which satisfies the condition A049820(k) = 4, namely 8 (8 - d(8) = 4).
a(5) = 1, as the only term in A045765 satisfying the condition is 7, as 7 - d(7) = 5.
a(22) = 2, as there are exactly two terms in A045765 satisfying the condition, namely 25 and 28, as 25 - d(25) = 28 - d(28) = 22.

Antti Karttunen, Table of n, a(n) for n = 0..65538

Cf. A000005, A045765, A049820, A060990, A082284, A262686.

Cf. A262901 (indices of nonzero terms), A262902.

(define (A262900 n) (let loop ((s 0) (k (A262686 n))) (cond ((<= k n) s) ((= n (A049820 k)) (loop (+ s (if (zero? (A060990 k)) 1 0)) (- k 1))) (else (loop s (- k 1))))))

a(n) = Sum_{k = A082284(n) .. A262686(n)} [A049820(k) = n] * [A060990(k) = 0].

In the above formula [ ] stands for Iverson bracket, giving in the first instance as its result 1 only when A049820(k) = n (that is, when k is really a child of n), and 0 otherwise, and in the second instance 1 only when A060990(k) = 0 (that is, when k itself has no children), and 0 otherwise. - Comment corrected by Antti Karttunen, Nov 27 2015

A263090 Primes p for which A049820(x) = p has a solution.

Original entry on oeis.org

2, 3, 5, 11, 17, 23, 29, 31, 41, 47, 53, 59, 61, 71, 73, 83, 89, 97, 101, 107, 127, 137, 139, 149, 151, 157, 173, 179, 181, 191, 197, 199, 211, 223, 227, 233, 239, 263, 269, 277, 281, 283, 311, 317, 331, 337, 347, 349, 367, 373, 389, 409, 419, 421, 431, 433, 443, 449, 457, 461, 467, 521, 523, 541, 547, 557, 569, 577, 587, 593, 599, 601, 607, 613, 617, 619, 631, 641, 643

Offset: 1

Antti Karttunen, Oct 11 2015

nonn

Primes p that there is at least one such k for which k - d(k) = p, where d(k) is the number of divisors of k (A000005).

			2 is present, as we have 6 - d(6) = 6 - 4 = 2.
3 is present, as we have 5 - d(5) = 3. The same holds for all lesser twin primes (A001359).

Antti Karttunen, Table of n, a(n) for n = 1..7955

Complement among primes: A263091.
Intersection of A000040 and A236562.
Cf. A001359 (a subsequence).
Cf. A000005, A049820, A060990.
Cf. also A263094.

lim = 10000; s = Select[Sort@ DeleteDuplicates@ Table[n - DivisorSigma[0, n], {n, lim}], PrimeQ]; Take[s, 79] (* Michael De Vlieger, Oct 13 2015 *)

allocatemem(123456789);
uplim1 = 2162160 + 320; \\ = A002182(41) + A002183(41).
v060990 = vector(uplim1);
for(n=3, uplim1, v060990[n-numdiv(n)]++);
A060990 = n -> if(!n,2,v060990[n]);
n=0; forprime(p=2, 131071, if((A060990(p) > 0), n++; write("b263090.txt", n, " ", p)));

;; With Antti Karttunen's IntSeq-library.
(define A263090 (MATCHING-POS 1 1 (lambda (n) (and (= 1 (A010051 n)) (not (zero? (A060990 n)))))))

A263094 Squares in A236562; numbers n^2 such that there is at least one such k for which k - d(k) = n^2, where d(k) is the number of divisors of k (A000005).

Original entry on oeis.org

0, 1, 4, 9, 16, 81, 121, 144, 169, 225, 289, 361, 441, 529, 576, 625, 841, 900, 961, 1024, 1089, 1296, 1444, 1521, 1600, 1681, 1764, 1849, 1936, 2025, 2304, 2401, 2601, 2704, 2809, 3025, 3249, 3721, 3969, 4096, 4225, 4356, 4624, 4761, 4900, 5041, 5184, 5476, 5625, 5776, 5929, 6241, 6400, 6561, 6889, 7056, 7396, 7569, 7744, 8281, 8464, 8649, 9216, 9409, 9801, 10201, 10404, 11025

Offset: 0

Antti Karttunen, Oct 11 2015

nonn

Starting offset is zero, because a(0)=0 is a special case in this sequence.

Antti Karttunen, Table of n, a(n) for n = 0..20763

Cf. A000005, A049820, A060990.
Intersection of A000290 and A236562.
Cf. A263092 (gives the square roots of these terms).
Cf. A263095 (complement among squares).
Cf. A262514 (a subsequence).
Cf. also A263090, A263098.

Take[Select[Sort@ DeleteDuplicates@ Table[n - DivisorSigma[0, n], {n, 20000}], IntegerQ@ Sqrt@ # &], 68] (* Michael De Vlieger, Oct 13 2015 *)

PARI
```
\\ See code in A263092.
```

(define (A263094 n) (A000290 (A263092 n)))

a(n) = A000290(A263092(n)).

A266111 If A082284(n) = 0, a(n) = 1, otherwise a(n) = 1 + a(A082284(n)), where A082284(n) = smallest number k such that k - d(k) = n, or 0 if no such number exists, and d(n) = the number of divisors of n (A000005).

Original entry on oeis.org

5, 4, 5, 3, 2, 2, 4, 1, 1, 3, 3, 2, 4, 1, 2, 3, 2, 2, 3, 1, 1, 7, 2, 6, 1, 1, 3, 5, 1, 4, 5, 3, 2, 1, 4, 2, 1, 1, 3, 3, 1, 2, 3, 1, 2, 9, 2, 8, 2, 1, 1, 7, 1, 6, 4, 1, 1, 5, 3, 4, 8, 3, 2, 1, 1, 2, 1, 1, 1, 5, 2, 4, 7, 3, 1, 1, 9, 2, 5, 1, 2, 8, 4, 7, 6, 1, 3, 6, 1, 5, 13, 6, 2, 4, 12, 5, 5, 4, 1, 3, 1, 2, 11, 1, 4, 3, 10, 2, 1, 1, 2, 2, 1, 1, 9, 3, 1, 1, 8, 2, 3, 7

Offset: 0

Antti Karttunen, Dec 21 2015

nonn

			Starting from n = 21, we get the following chain: 21 -> 23 -> 27 -> 29 -> 31 -> 35 -> 37, with A082284 iterated 6 times before the final term 37 (for which A060990(37) = A082284(37) = 0) is encountered. Here we count the terms (not steps) in whole chain, thus a(21) = 7.

Antti Karttunen, Table of n, a(n) for n = 0..10000

One more than A266110.
Number of significant terms on row n of A265751 (without its trailing zeros).
Cf. tree A263267 (and its illustration).
Cf. A000005, A060990, A082284.
Cf. also A264971.

If A060990(n) = 0, a(n) = 1, otherwise a(n) = 1 + a(A082284(n)).
Other identities. For all n >= 0:
a(n) = 1 + A266110(n).

A330738 Ordinal transform of A049820, where A049820(n) = n - d(n), with d(n) the number of divisors of n (A000005).

Original entry on oeis.org

1, 2, 1, 2, 1, 1, 1, 1, 1, 2, 1, 3, 1, 1, 2, 3, 1, 1, 1, 1, 2, 1, 1, 1, 1, 2, 1, 3, 1, 4, 1, 1, 2, 1, 1, 2, 1, 1, 2, 1, 1, 2, 1, 1, 2, 1, 1, 2, 1, 1, 1, 2, 1, 3, 2, 1, 1, 1, 1, 2, 1, 1, 2, 3, 1, 2, 1, 1, 2, 2, 1, 1, 1, 1, 2, 2, 1, 3, 1, 4, 1, 1, 1, 1, 2, 1, 1, 1, 1, 2, 2, 1, 1, 1, 1, 1, 1, 1, 1, 2, 1, 1, 1, 1, 1

Offset: 1

Antti Karttunen, Jan 11 2020

nonn

Antti Karttunen, Table of n, a(n) for n = 1..65537

Cf. A000005, A049820, A060990, A155043.

b[_] = 0;
a[n_] := a[n] = With[{t = n - DivisorSigma[0, n]}, b[t] = b[t]+1];
Array[a, 105] (* Jean-François Alcover, Dec 22 2021 *)

up_to = 65537;
ordinal_transform(invec) = { my(om = Map(), outvec = vector(length(invec)), pt); for(i=1, length(invec), if(mapisdefined(om,invec[i]), pt = mapget(om, invec[i]), pt = 0); outvec[i] = (1+pt); mapput(om,invec[i],(1+pt))); outvec; };
A049820(n) = (n-numdiv(n));
v330738 = ordinal_transform(vector(up_to, n, A049820(n)));
A330738(n) = v330738[n];

A348093 Numbers k >= 1 such that there is no pair (x,y) such that x - d(x) = k or y + d(y) = k, where d = A000005 = number of divisors.

Original entry on oeis.org

8, 20, 36, 40, 67, 68, 79, 88, 100, 116, 117, 131, 132, 134, 140, 156, 164, 167, 180, 185, 196, 204, 228, 244, 252, 268, 276, 284, 300, 308, 312, 321, 324, 341, 348, 370, 372, 379, 388, 401, 405, 408, 420, 425, 436, 439, 453, 460, 476, 479

Offset: 1

Ctibor O. Zizka, Sep 29 2021

nonn

Numbers k >= 1 such that A060990(k) + A036431(k) = 0.

			k = 8 is a term: there are no x,y such that x - d(x) = 8, y + d(y) = 8.

Amiram Eldar, Table of n, a(n) for n = 1..10000

Cf. A000005, A036431, A049820, A060990, A062249.

Intersection of A036434 and A045765.

With[{max = 480}, Complement[Range[max], Select[Union[Flatten[Table[n + DivisorSigma[0, n]*{-1, 1}, {n, 1, max + 2 + 2*Ceiling[Sqrt[2*max+4]]}]]], # <= max &]]] (* Amiram Eldar, Mar 04 2023 *)

okp(k) = sum(i=1, k, i+numdiv(i) == k) == 0;
okm(k) = sum(i=1, 2*k+2, i-numdiv(i) == k) == 0;
isok(k) = okp(k) && okm(k); \\ Michel Marcus, Oct 01 2021

cp's OEIS Frontend

A266110 If A082284(n) = 0, a(n) = 0, otherwise a(n) = 1 + a(A082284(n)), where A082284(n) = smallest number k such that k - d(k) = n, or 0 if no such number exists, and d(n) = the number of divisors of n (A000005).

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A266116 The last nonzero term on each row of A265751.

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A262900 a(n) = number of leaf-children n has in the tree generated by edge-relation A049820(child) = parent.

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A263090 Primes p for which A049820(x) = p has a solution.

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A263094 Squares in A236562; numbers n^2 such that there is at least one such k for which k - d(k) = n^2, where d(k) is the number of divisors of k (A000005).

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A266111 If A082284(n) = 0, a(n) = 1, otherwise a(n) = 1 + a(A082284(n)), where A082284(n) = smallest number k such that k - d(k) = n, or 0 if no such number exists, and d(n) = the number of divisors of n (A000005).

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A330738 Ordinal transform of A049820, where A049820(n) = n - d(n), with d(n) the number of divisors of n (A000005).

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A348093 Numbers k >= 1 such that there is no pair (x,y) such that x - d(x) = k or y + d(y) = k, where d = A000005 = number of divisors.

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