## Abstract

The distribution of the number of academic publications against citation count for papers published in the same year is remarkably similar from year to year. We characterise the shape of such distributions by a ‘width’, \(\sigma ^2\), associated with fitting a log-normal to each distribution, and find the width to be approximately constant for publications published in different years. This similarity is not surprising, after all, why would papers in a given year be cited more than another year? Nevertheless, we show that simple citation models fail to capture this behaviour. We then provide a simple three parameter citation network model which can reproduce the correct width over time. We use the citation network of papers from the hep-th section of arXiv to test our model. Our final model reproduces the data’s observed ‘width’ when around 20 % of the citations in the model are made to recently published papers in the entire network (‘global information’). The remaining 80 % of citations are made using the references from these papers’ bibliographies (‘local searches’). We note that this is consistent with other studies, though our motivation to achieve the above distribution with time is very different. Finally, we find that, in the citation network model, varying the number of papers referenced by a new publication is important as it alters the parameters in the model which are fitted to the data. This is not addressed in current models and needs further work.

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## Notes

- 1.
We omit year 2003 from our analysis because it is incomplete.

- 2.
The bin scale was chosen to ensure there were no empty bins below the bin containing the highest citation values.

- 3.
There is a small correction to this form due to the possibility of that the same vertex

*i*could be chosen more than once which is excluded in the actual model. - 4.
The full Price model may be solved exactly within the mean-field approximation in the infinite time limit. Those solutions are very close to numerical results found for finite sized simulations such as ours. For \(\langle k^{({\text {in}})} \rangle =12.0\), these formulae give \(z=0.156\) if \(p=0.55\) and we find we need \(p=0.59\) in order to get the same value of \(z=0.169\) found in our data. However, we remove the first thousand papers created in our simulation so we do not expect an exact match with the theoretical expressions.

- 5.
Again there is a small correction to this form to allow for the fact that we do not allow the same vertex

*i*to be chosen more than in model B. - 6.
This is different from the ‘half-life’ values referred to later, which are

*measured*from the data. - 7.
Note this is why we call it the

*median half-life*: if you plot the number of citations gained by a paper against year and take the median, this is the value we call the median half-life \(T_{\text {med}}\). Thus the value of \(T_{\text {med}}\) for a given paper increases over time. We call it the*median half-life*not a*half-life*because the*half-life*, as defined in a process with an exponential decay, is a fixed value, only equal to our median half-life in the limit of an infinitely old paper. We expect any estimate of the half-life of a paper’s citations to be roughly constant whereas our*median half-life*\(T_{\text {med}}\) measurement varies from year to year, increasing until it reaches the formal half-life value. - 8.
- 9.
For model C our final comparison tool is the \(\sigma ^2\) plot of model C and the hep-th data. Note that so far we have used

*z*and the number of core papers*C*as our comparison tools. - 10.
This estimate assumes no correlation between in- and out-degree as a better estimate for the average in-degree of core papers chosen using cumulative advantage is \(\langle (k^{({\text {in}})})^2 \rangle /\langle k^{({\text {in}})} \rangle\).

- 11.
We have also done another check. The proportion of core papers referenced per new publication is therefore, on average \(=\frac{C}{\langle k^{({\text {out}})} \rangle }=\frac{C}{C(1+q\langle k^{({\text {out}})} \rangle )}=\frac{3.9}{12.0} \approx 0.3\) for \(q=0.17\) or \(q=0.20\) (which are the mathematically expected value of

*q*and the*q*derived from fitting the model C to the hep-th data, respectively). In*A Mathematical Theory of Citing*and Solla Price (1965) this was found to be 0.1 and 0.15 for their models, respectively. Again, our values are consistent with these as they are low.

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## Acknowledgments

We would like to thank James Gollings and James Clough for allowing us to use their transitive reduction code from which we created our own declustering code. We would like to thank Tamar Loach for sharing her results on related projects and M. V. Simkin for discussions about his work.

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## Appendix

### Appendix

### Fitting procedure

We follow the procedure used in Evans et al. (2012) and Goldberg and Evans (2012). We use logarithmic binning so that the citations in bin b \(c_b \in {\mathbb {Z}}\) with \(c_{b+1}\) equal to \(R c_b\) rounded to the nearest integer or to \((c_{b}+1)\), whichever is the highest, where *R* is some fixed bin scale chosen to ensure there are no empty bins below the bin containing the highest citation values. The edge of the first bin is chosen to be the lowest integer above the value \(0.1 \langle c \rangle\). In order to make the fit we compare the total value in the *b*th bin, \(n_b= \sum _{c=c_{b}}^{c_{b+1}} n(c)\), against the expected value

This gives us a sequence of data and model values which are compared using a non-linear least squares algorithm to give us values for \(\sigma ^2, A\) and *B*.

### Lognormal out-degree distribution

In all above models the citation networks were created by determining the number of references a new node would create (via a normal distribution mean 12.0, standard deviation 3.0 references) and then having a method of deciding which nodes to reference. However, we found that a lognormal fits the out-degree distribution of the hep-th data better than a normal distribution, Figs. 4 and 21, respectively.

As further work we inputted this fitted lognormal to determine the number of references created by a new node into the model C and ran it for our final parameters \((p,q)=(0.55,0.20)\) and \(\tau = 200\)papers. The ratio of the \(\sigma ^2\) values associated with the in-degree distribution of papers published in the same year for the data is divided by the corresponding year’s \(\sigma ^2\) for this modified model C and plotted against year in Fig. 22. We find that the ratio is close to 1, however, it is not as close as the original model C, Fig. 20. Therefore the \(\sigma ^2\) plot *does* depend on in-degree, contradicting (Ren et al. 2012), who say it is ‘innocuous’ to the in-degree distribution of the citation network. Although this out-degree distribution has been observed by the literature (Vázquez 2001) its use in a citation network model is novel and original.

Although the \(\sigma ^2\) plot is lower than that of the original model C we conjecture that by varying the \(\tau\) of the model C the \(\sigma ^2\) plot could match that of the data’s, this may also increase the attention span to something closer to a year as expected by Simkin and Roychowdhury (2007).

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Goldberg, S.R., Anthony, H. & Evans, T.S. Modelling citation networks.
*Scientometrics* **105, **1577–1604 (2015). https://doi.org/10.1007/s11192-015-1737-9

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### Keywords

- Complex networks
- Directed acyclic graphs
- Bibliometrics
- Citation networks

### Mathematics Subject Classification

- 91D30